The sweet potatoes should mash into a soft, creamy base, but the marshmallows are what need your attention at the end. Put them on too early and they can darken before the filling is ready; add them after the first bake and they have just enough time to puff and turn golden. That two-stage bake is what makes this sweet potato casserole with marshmallows worth making from scratch.
I boil peeled sweet potato cubes until a fork slips through easily, then mash them with butter, brown sugar, milk, vanilla, salt, and cinnamon. Beating in the eggs after the potatoes are mashed gives the filling a gentle set without losing its smooth texture. It’s a straightforward casserole, but each stage has a clear cue, from tender potatoes to a set center and a toasted marshmallow top.
The finished casserole is soft beneath the topping, with a warm hint of cinnamon and a golden layer of marshmallows that gives way to the spoon.
Kitchen Note
After draining, let the potatoes cool in the pot for the full 5 minutes before mashing. That brief pause takes the edge off their heat, so the beaten eggs blend into the filling instead of starting to cook as soon as they go in.
Save this sweet potato casserole with marshmallows for its creamy center and golden, puffed topping.
Why the Marshmallows Go on After the Sweet Potatoes Bake
The filling and topping need different amounts of oven time. The sweet potato mixture has to bake long enough for the eggs to set the center; mini marshmallows only need a short finish to puff and turn golden. Baking them together from the start gives the marshmallows far more time in the oven than they need, so their color can race ahead while the casserole is still baking.
Instead, bake the filling first, then scatter the marshmallows over the surface. That order gives you a distinct layer: a smooth, set sweet potato base underneath and a soft, toasted topping above it. Keep an eye on the marshmallows during their final bake. Their color is a more useful visual cue than waiting for them to look deeply browned, since the target here is puffed and golden.
The center matters, too. A browned top alone doesn’t tell you whether the filling is ready, so check the center of the casserole separately. Once it’s set and reaches 160°F, the sweet potato layer is ready for its marshmallow finish.
What Gives This Casserole Its Soft, Set Filling
- Sweet potatoes — Fresh sweet potatoes give the filling its body and natural sweetness. Choose firm potatoes without soft spots, then peel and cut them into evenly sized cubes so they become tender at about the same time. Their moisture varies, so drain them well before mashing.
- Butter — Butter enriches the mash and helps it feel creamy rather than dry. Unsalted butter lets the salt in the filling stay balanced. Keep the tablespoon used to grease the baking dish separate from the butter that goes into the potatoes.
- Light brown sugar — Its mild molasses note rounds out the sweet potatoes without turning the filling into a syrupy layer. Pack it into the measuring cup so the amount is consistent.
- Whole milk — Milk loosens the mash and makes it easier to spread. Whole milk contributes a fuller texture; lower-fat milk will make the filling less rich.
- Eggs — The lightly beaten eggs help the filling set during its first bake. Stir them in after mashing so they’re evenly distributed through the potatoes.
- Vanilla and cinnamon — Vanilla softens the earthy sweetness, while cinnamon brings gentle warmth. Fresh, fragrant ground cinnamon makes the difference here; the vanilla doesn’t need to be an expensive specialty extract.
- Mini marshmallows — Their small size makes it easier to cover the casserole in an even layer, and they puff during the short final bake. Spread them across the surface rather than piling them in the middle.
Getting the Sweet Potato Layer Set Before the Topping
Simmer the cubes until tender throughout
Heat the oven to 350°F and grease a 9×13-inch baking dish with 1 tablespoon of the butter, keeping the remaining butter for the filling. Put the sweet potato cubes in a large pot and cover them with cold water by 1 inch. Bring the water to a boil over high heat, then lower the heat and simmer for 12 to 15 minutes. A fork should pass through a cube easily; if the center still feels firm, give the potatoes more time within that range rather than mashing them undercooked.
Mash the filling until smooth
Drain the potatoes and return them to the pot. After their brief cooling pause, mash with the remaining butter, brown sugar, milk, vanilla, salt, and cinnamon until the mixture is smooth. Scrape around the bottom and sides of the pot as you mash, where firmer pieces can hide. Stir in the lightly beaten eggs until they’re evenly combined, then spread the filling in the prepared dish.
Bake the filling before adding marshmallows
Bake the casserole for 25 to 30 minutes, until the center is set and reaches 160°F. If the center still looks loose, give it more time before adding the topping; the marshmallow bake is not a substitute for setting the filling. Scatter the mini marshmallows evenly over the surface and bake for 8 to 12 minutes, until puffed and golden. Rest the casserole for 10 minutes before serving so the filling settles and portions are easier to scoop.
Small Changes to the Texture and Topping
Leave the mash slightly rustic
For a more textured base, stop mashing when a few small, tender pieces remain instead of working the potatoes completely smooth. Keep the rest of the ingredients and baking method the same; the result will feel more like mashed sweet potatoes under the marshmallows than a uniform purée.
Make the marshmallow layer more prominent
For extra marshmallow in each bite, fold half of the package into the finished sweet potato filling and scatter the other half over the top after the first bake. The marshmallows inside soften into the filling, while the ones on top still puff and brown. This shifts the casserole sweeter and stickier, so keep the topping spread in an even layer.
Storage and Reheating
- Refrigerator: Refrigerate leftovers in a covered container for 3 to 4 days. The marshmallows soften and lose their toasted texture.
- Freezer: Freezing the finished casserole isn’t ideal; the marshmallow topping turns sticky and the filling may lose some of its smooth texture after thawing.
- Reheating: Warm portions in a 325°F oven until heated through to 165°F. Cover loosely with foil if the marshmallow topping is already dark; reheating uncovered can brown it further before the center is hot.
Questions That Come Up When Making This Casserole
Sweet Potato Casserole with Marshmallows
Ingredients
Equipment
Method
- Preheat the oven to 350°F. Grease a 9x13-inch baking dish with 1 tablespoon of the butter and reserve the remaining 7 tablespoons.
- Put the sweet potatoes in a large pot and cover with cold water by 1 inch.
- Bring the water to a boil over high heat, then reduce the heat and simmer for 12 to 15 minutes, until fork-tender.
- Drain the sweet potatoes, return them to the pot, and let them cool for 5 minutes.
- Mash the sweet potatoes with the remaining butter, brown sugar, milk, vanilla, salt, and cinnamon until smooth.
- Stir in the beaten eggs until evenly combined, then spread the mixture in the prepared baking dish.
- Bake for 25 to 30 minutes, until the center reaches 160°F and is set.
- Scatter the marshmallows evenly over the casserole and bake for 8 to 12 minutes, until puffed and golden.
- Let the casserole rest for 10 minutes before serving.