Sweet Potato Casserole Recipe: Roasted, Pecan or Marshmallow

What makes this sweet potato casserole different

Sweet potato casserole is the Thanksgiving side that sits halfway to dessert: mashed sweet potatoes sweetened with brown sugar and butter, bound with eggs, spread in a baking dish and finished with a pecan streusel, toasted marshmallows, or both. Most recipes boil the potatoes, bake until the topping looks right, and leave it there.

This one changes three things. The potatoes are roasted whole, so the filling is sweeter and less watery before any sugar goes in. The filling is baked to a number, 165°F (74°C) in the center, because it is an egg custard at heart and a thermometer is the only honest test of an egg dish. And the topping is treated as a choice with consequences, with exact amounts for a pecan streusel, a marshmallow lid, or both, and the calorie math for each.

For one 9x13-inch (23x33 cm) dish serving 10, you need about 3 1/2 lb (1.6 kg) of sweet potatoes (5 cups or 1 kg of roasted flesh), butter, brown sugar, 2 eggs, 1/2 cup (120 ml) of milk, salt, vanilla, warm spice, orange zest and a little lemon juice, plus the topping. Allow about 2 1/2 hours, most of it hands-off roasting and baking, or split it across two days with the make-ahead plan below.

Roast, don’t boil: why the oven makes a sweeter casserole

A sweet potato gets much of its sweetness in the oven. Raw sweet potato contains starch and some sucrose, but no maltose at all: USDA nutrient data lists 0 g of maltose and 12.65 g of starch per 100 g raw, against 3.12 g of maltose and 7.05 g of starch per 100 g of flesh baked in the skin. More than 40% of the starch has been broken down, and maltose, zero in the raw root, is now almost half the sugar. The worker is the potato’s own enzyme, beta-amylase, which can only attack starch once the granules have swollen with heat, and which is destroyed as the flesh gets hotter still.

The window usually quoted, after Harold McGee, runs from about 135°F to 170°F (57°C to 77°C). Lab work suggests that is conservative at the top: in a 1994 study of the classic US variety ‘Jewel’, maltose kept rising to a peak at 176°F (80°C). Either way, the rule is the same. The longer the flesh spends in that range, the sweeter it gets, and maltose is also the raw material for the toasty, caramel aroma of a baked sweet potato. Serious Eats’ Food Lab found that holding potatoes in a 150°F (66°C) water bath for an hour before roasting made them markedly sweeter and more flavorful; starting our potatoes in a cold oven is a gentler version of the same idea.

Other methods lose on both counts. Boiled flesh takes on water: America’s Test Kitchen found boiled sweet potatoes “waterlogged” and fixed it by stirring them in a dry pot for 7 minutes, which drove off 12% of their weight. Boiling also heats the flesh so fast that the enzyme has little time to work. A microwave is worse: brief microwave heating blocked about 80% of maltose formation in the 1994 study, and ATK found microwaved pieces gave a “leathery, tough mash”. Canned sweet potatoes, heated fast in the can, stay higher in starch, which a 1980 Journal of Food Science study put down to the rapid inactivation of the enzyme. That is why a roasted casserole needs less sugar than a boiled one to taste as sweet.

Marshmallow or pecan streusel? An honest comparison

The marshmallow lid is the lighter topping. On this recipe’s filling (about 195 kcal a serving), 4 cups (180 g) of mini marshmallows add about 60 kcal a serving, while the pecan streusel adds about 204 kcal. The streusel’s calories are butter and nuts, not sugar: its 1 cup (109 g) of pecans alone is 753 kcal. The marshmallows bring almost pure sugar, about 10 g a serving, where the streusel brings crunch, fat and a toasted, savory edge that balances the sweet filling.

They also behave differently in the oven. A marshmallow is a foam of sugar syrup and air held up by gelatin; in the heat the gelatin melts, the air expands and it puffs, then the sugar browns. Left too long, the bubbles burst, the lid collapses and the peaks burn, which is why the marshmallows go on only for the last 8 to 10 minutes. A streusel needs the whole bake to crisp, which is why it goes on at the start, cold. For both, bake with the streusel and add marshmallows to the gaps at the end.

The best-known restaurant version, the sweet potato casserole at Ruth’s Chris Steak House (founded in New Orleans in 1965), takes the pecan side: a brown-sugar, flour and pecan crust over a sweetened, egg-bound filling, and no marshmallows. Its official menu lists the side at 880 calories, more than twice the calories of a serving of this recipe with streusel. Accounts disagree about who created it, so we leave that open.

Who prefers which is less settled than the internet suggests. An AP-NORC poll in November 2023 found that 42% of Northeasterners chose sweet potatoes with no marshmallows, against 29% in both the Midwest and the South; nationally, among adults who celebrate Thanksgiving, about a third preferred no marshmallows and about a quarter wanted them. We could find no poll that pits marshmallow against pecan, so the familiar claim that the South is pecan country and the North is marshmallow country is anecdote, not data.

Why is sweet potato casserole watery? The causes, ranked

1. Boiled potatoes. They absorb water in the pot, and that water comes out in the oven. Roast instead, or, if you must boil, stir the drained potatoes in the dry pot over medium heat until the steam stops, America’s Test Kitchen’s fix.

2. Too much milk. Sweet potatoes already carry more moisture and less starch than white potatoes, so recipes that pour in cream as if for mashed russets end up loose. This recipe uses 1/2 cup (120 ml) of milk for 5 cups (1 kg) of flesh.

3. An underbaked center. The eggs are what set the filling, and they need heat to do it: a whole egg begins to set at 144°F to 158°F (62°C to 70°C), and sugar pushes that higher. Bake until the center reads at least 165°F (74°C).

4. An overbaked center. Past about 185°F (85°C) the egg proteins tighten, and a few degrees higher they curdle and squeeze liquid out, so a casserole baked far too long can weep just like one baked too little.

5. Undrained canned yams. The syrup in the can is liquid you did not plan for. Drain them well and cut back the sugar.

6. Serving it straight from the oven, or covering it while hot. Give it 10 to 15 minutes to set, uncovered, so steam escapes instead of condensing back onto the top. Do not try to rescue a baked casserole by stirring in flour or cornstarch; it will taste raw and pasty.

The make-ahead plan for Thanksgiving

Up to 1 week ahead: make the streusel and keep it in a covered container in the fridge. Held cold and apart from the filling, it stays crisp-ready; mixed onto the filling in advance, it drinks up moisture and bakes soft.

Up to 2 days ahead: roast, scoop and mash the potatoes with the butter only, spread the mash in a shallow container (or straight into the buttered baking dish) and refrigerate it within 2 hours, uncovered until cool, then covered. Plain cooked sweet potato is the safest thing to hold, and two days fits inside both the US and the UK guidance for cooked food. This is our preferred plan, because it keeps raw egg out of the fridge altogether.

Up to 1 day ahead, if you prefer: finish the whole filling, eggs and all, spread it in the dish, cover and refrigerate. Keep the topping separate.

On the day: beat the seasoning, eggs and milk straight into the cold mash; there is no need to warm it. Fill the dish, top it and bake, allowing 15 to 20 minutes more than for a freshly made filling; the thermometer, not the clock, tells you when it is done.

To freeze: freeze the plain mash, or the baked casserole without its topping, wrapped airtight, for 2 to 3 months. Thaw in the fridge overnight and pour off any liquid that has separated (freezing ruptures the potato’s cells). For plain mash, beat in the seasoning, eggs and milk as in the recipe. Add a fresh topping and bake until the center reaches 165°F (74°C) either way. Streusel freezes well on its own; marshmallows do not, so add them fresh. To reheat leftovers, cover with foil and warm at 350°F (175°C) for 20 to 25 minutes (longer for a full fridge-cold dish), until the center is back to 165°F (74°C); a fresh handful of marshmallows for the last 5 minutes, uncovered, restores the top.

The history: marshmallows arrived before the booklet everyone credits

Read almost any account of this dish and you will be told that it began in 1917, when the makers of Angelus Marshmallows hired the Boston cookery writer Janet McKenzie Hill to write a recipe booklet containing “the first documented appearance” of sweet potatoes baked under marshmallows. That phrase comes from a 2011 Saveur article, which gives no title and quotes no recipe text from the booklet; later articles attribute it to the Library of Congress, but the Library’s blog added the claim in 2012 by quoting Saveur. The booklet is real, a 16-page pamphlet copyrighted in 1917 by Rueckheim Bros. & Eckstein of Chicago (the firm did not become the Cracker Jack Company until 1922), yet we could not find its sweet potato recipe quoted anywhere.

The dish is in print earlier. On March 5, 1915, the Ward-Belmont Hyphen, the student weekly of Ward-Belmont School in Nashville, described a luncheon with “baked sweet potatoes with marshmallows” on the menu. In the summer of 1916, the Woman’s Club of Reading, Pennsylvania, compiled its Club House Cook Book, and Nellie Gery’s entry, “Sweet Potato and Marshmallow”, is the modern casserole in full: “Boil potatoes in unsalted water, until tender; remove the skins and mash as Irish potatoes, stir into them sugar, butter and salt to taste. Place in a buttered baking dish, cover the top with marshmallows. Put in the oven until the mashmallows [sic] brown and swell.” And it kept turning up in 1917, the booklet’s year: on January 5, Florida’s county home demonstration agents, meeting at the Florida State College for Women in Tallahassee, sat down to a banquet in the college dining room with “Marshmallow Sweet Potatoes” on the menu, and in its June and July issue Hill’s own magazine, American Cookery, printed a reader’s tip: “Place a marshmallow in center of each square and brown in oven to a nice rich brown.”

The earliest sighting we found is Southern, and so is the Tallahassee banquet, which undercuts another favorite line, that Southerners respected sweet potatoes too much to put candy on them. Some Southern writers did look down on it; Harriet Ross Colquitt’s Savannah Cook Book of 1933 found the plain version “far, far nicer”, as quoted by Saveur. But the dish was on Southern tables from the start. By November 1920 the Ladies’ Home Journal was printing “Mashed Sweet Potatoes With Marshmallows” on a Thanksgiving menu.

The base is much older. Mary Randolph’s The Virginia House-Wife (quoted here from the 1830 edition) has a sweet-potato pudding of boiled, sieved potatoes, eggs, sugar, butter, nutmeg, lemon and brandy, and adds: “Irish potato pudding is made in the same manner, but is not so good.” Lettice Bryan’s Kentucky Housewife (1839) bakes a crustless one, and prefers baked potatoes to parboiled ones for it, and Abby Fisher, in What Mrs. Fisher Knows About Old Southern Cooking (San Francisco, 1881), sharpened her sweet potato pie with the juice of an orange and half its grated peel, an idea this recipe borrows with its orange zest. Fannie Farmer’s 1896 Boston Cooking-School Cook Book already had glazed sweet potatoes, an early printed form of candied yams.

The name came later than the dish. The earliest English “Sweet Potato Casserole” we found, in the Washington Post of November 13, 1927, is layered sweet potatoes, tart apples and sausage. In December 1942 a syndicated food column, Lynn Chambers’s “Lynn Says” (Western Newspaper Union; we read it in the Toledo Union Journal of December 24), printed a mashed, egg-lightened, marshmallow-topped sweet potato casserole and claimed it had taken “a mighty fine lead” in a poll of servicemen’s favorite foods. The pecan streusel we could not date reliably; it is everywhere in late-20th-century community cookbooks, and pecans were already going into Southern sweet potato dishes by the 1920s.

Yams, sweet potatoes and the world’s other sweet potato dishes

The orange “yams” in American stores are sweet potatoes (Ipomoea batatas, the morning-glory family). True yams (Dioscorea) are a different, starchier and drier tuber grown mostly in Africa; do not swap one for the other in this recipe. The Library of Congress notes that the USDA requires any “yam” label to say “sweet potato” as well, which is why a can of Bruce’s carries both words: “Yams” and “Cut Sweet Potatoes”. Louisiana is often said to have coined the word in 1937, when LSU horticulturists and the state’s growers began marketing its moist orange sweet potatoes nationally as “Louisiana yams”; they made it a national brand, but Americans had long used the word: Amelia Simmons listed “Pompkins, Potatoes or Yams” in 1796, and a 1918 Barrett Company booklet noted that some varieties “are also called ‘yams’ in the South and the name is now in general use”.

For this dish, buy the moist, orange-fleshed types: Covington, the most widely grown variety in North Carolina (the top US producer since 1971), LSU-bred Beauregard, Jewel or Garnet. White and Japanese sweet potatoes such as Hannah or Murasaki are drier and paler; they mash stiffer and need more milk.

The same root is a festival food around the world. Japan fries it and tosses it in syrup and sesame as daigaku imo, “university potato”, a name with several competing origin stories, most of them set among Tokyo university students between the 1910s and the early 1930s. Okinawa, where the sweet potato arrived from Fujian in 1605, grows the purple beni imo. Korea owes its goguma to the envoy Jo Eom, who saw the plant on Tsushima in 1763 and sent seed tubers home to Busan. New Zealand’s kūmara has been grown there since first settlement, around the 13th century; most researchers hold that people carried the plant from South America into Polynesia around AD 1000 to 1200, though whether Polynesians sailed east or South Americans sailed west is still debated; a 2018 genetic study argued for natural spread and has been sharply disputed. In Lima, camote sits beside ceviche and goes into picarones, the syrup-soaked sweet potato and squash doughnuts already named in print in 1798.