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When a fork slides through a sweet potato cube with no resistance, the pieces are ready; firm centers are what leave little lumps in the mash. I cook the cubes until they’re genuinely tender, then mash them with butter while they’re still hot. That gives this sweet potato casserole with marshmallows a smooth, rich base without needing a mixer.

The filling gets a short pause before the eggs go in, and that small bit of patience makes the mixing easier. Brown sugar and cinnamon bring warmth without burying the sweet potatoes, while the marshmallows go on near the end so they puff and turn lightly golden instead of disappearing into the casserole. The finished center is soft and gently set beneath a pillowy, toasted top.

Why the Eggs Matter More Than the Marshmallow Crown

The marshmallows get the attention, but the eggs are what give the sweet potato filling its gentle, sliceable set. Without them, the mash stays closer to a side of mashed sweet potatoes: soft, spoonable, and less cohesive after baking. They don’t turn the casserole into a firm custard. Instead, they help the filling hold together while keeping the middle tender.

That balance is useful because sweet potatoes already bring plenty of body. There’s no need to add flour or another thickener, which would make the filling heavier. The eggs also give the casserole a clean contrast with its topping: a soft, smooth layer underneath and airy marshmallows above. Once the center reaches 160°F, the base is set enough to serve after its rest without losing the creamy texture that makes this casserole comforting.

What Sweet Potatoes, Brown Sugar, and Mini Marshmallows Each Bring

• Sweet potatoes — Choose firm potatoes without soft spots; fresh ones give you control over the tenderness and moisture of the mash. Peel them and cut the cubes evenly so they cook at a similar pace. If you can’t find orange-fleshed sweet potatoes, another variety will still work, though the color and sweetness may differ.
• Butter and milk — Butter adds richness as it melts into the hot potatoes, and milk loosens the mash into a soft filling. Any standard dairy milk works; whole milk makes it a little richer, while lower-fat milk keeps the texture lighter. For a dairy-free version, plant-based butter and unsweetened oat milk are practical swaps.
• Light brown sugar — Its mild molasses note rounds out the sweet potatoes without making the filling taste like candy. Dark brown sugar works in the same amount for a deeper, more pronounced molasses flavor; granulated sugar can replace it if needed, but the filling loses that warm caramel note.
• Eggs — They help the filling set as it bakes, so the casserole serves neatly instead of spreading like loose mash. Lightly beating them before adding helps them mix through the potatoes evenly. A commercial baking egg replacer made for replacing two eggs is the most workable egg-free option, though the filling won’t be quite as rich or cohesive.
• Cinnamon and vanilla — Cinnamon gives the filling a familiar warmth, and vanilla softens the brown sugar’s sharper edge. Use whichever vanilla extract you keep on hand; if cinnamon isn’t your preference, leaving it out keeps the sweet potato flavor more forward.
• Mini marshmallows — Their small size makes it easy to cover the casserole evenly, and they puff quickly at the end of baking. Fresh marshmallows brown more evenly than a bag that has gone sticky or stale. Large marshmallows can be cut into smaller pieces if minis aren’t available.

From Fork-Tender Cubes to a Toasted Marshmallow Top

Boil the cubes until they mash cleanly

Heat the oven to 350°F and grease a 9×13-inch baking dish. Put the peeled, 1-inch sweet potato cubes in a 5-quart saucepan, cover them with water, and bring the water to a boil over high heat. Reduce the heat and simmer for 15 to 20 minutes, until a fork pierces the cubes with little resistance. Drain them well, return them to the pan, and mash with the butter until smooth; if you leave excess water clinging to the potatoes, the filling can turn loose.

Mix the filling without overworking it

Give the mashed potatoes five minutes to cool, then stir in the milk, brown sugar, salt, cinnamon, and vanilla. Mix until the seasonings and milk are distributed and the filling looks even. Stir in the lightly beaten eggs until combined, stopping once there are no visible streaks. Spread the mixture into the prepared baking dish in an even layer so the center and edges bake at a similar rate.

Bake the center before adding the marshmallows

Bake the filling for 20 to 25 minutes, until the center reaches 160°F. Scatter the mini marshmallows evenly across the surface; a bare patch will stay uncovered while crowded areas can brown faster. Return the dish to the oven for 5 to 8 minutes, watching for puffed marshmallows with lightly golden tops. Rest the casserole for 10 minutes before serving so the filling has a chance to settle.

Small Changes That Keep This Sweet Potato Casserole Familiar

Questions That Come Up With Marshmallow-Topped Sweet Potato Casserole

Sweet Potato Casserole with Marshmallows

Sweet potato casserole with marshmallows bakes into a smooth, cinnamon-scented filling topped with puffed, lightly golden marshmallows. Boil and mash the sweet potatoes, then finish the casserole in a 9x13-inch baking dish.

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Prep Time 20 minutes mins

Cook Time 47 minutes mins

Resting 10 minutes mins

Total Time 1 hour hr 17 minutes mins

Servings: 8 servings

Course: Side Dish

Cuisine: American

Calories: 422

Ingredients Equipment Method Notes

Ingredients

  

For the sweet potato filling

• 3 lb sweet potatoes peeled and cut into 1-inch cubes
• 4 tbsp unsalted butter
• 0.5 cup milk
• 0.5 cup light brown sugar packed
• 0.5 tsp salt
• 0.5 tsp ground cinnamon
• 1 tsp vanilla extract
• 2 eggs large, lightly beaten

For the topping

• 1 package (10 oz) mini marshmallows

Equipment

• 1 9x13-inch baking dish
• 1 5-quart saucepan

Method

 

Make the filling

1. Heat the oven to 350°F.
2. Grease a 9x13-inch baking dish.
3. Put the sweet potatoes in a 5-quart saucepan and cover with water.
4. Bring to a boil over high heat.
5. Reduce the heat and simmer for 15 to 20 minutes, until very tender when pierced with a fork.
6. Drain the sweet potatoes well and return them to the saucepan.
7. Mash the sweet potatoes with the butter until smooth.
8. Let the potatoes cool for 5 minutes.
9. Stir in the milk, brown sugar, salt, cinnamon, and vanilla.
10. Mix in the eggs until combined.

Assemble and bake

1. Spread the sweet potato mixture in the prepared dish.
2. Bake for 20 to 25 minutes, until the center reaches 160°F.
3. Scatter the marshmallows evenly over the casserole.
4. Bake for 5 to 8 minutes, until puffed and lightly golden.
5. Rest the casserole for 10 minutes before serving.

Notes

For the smoothest filling, mash the potatoes while they are hot. Refrigerate leftovers for 3 to 4 days; freezing is suitable. For a dairy-free variation, use dairy-free butter and unsweetened plant milk.