As the mini marshmallows puff and turn golden, the cinnamon-scented sweet potato filling underneath is already warm and set. That contrast is what I want from sweet potato casserole with marshmallows and cinnamon: a soft, gently sweet center with a toasted top that gives way under the spoon.
I keep the filling smooth enough to feel creamy, but not so overworked that it loses its body. Light brown sugar brings a mellow sweetness, and cinnamon gives the sweet potatoes a warm flavor without taking over. The marshmallows go on only after the filling has had its first bake, so they get their own short turn in the oven and stay visible on top.
Kitchen Note
Give the mashed sweet potatoes five minutes to cool before stirring in the egg. That short pause helps the egg blend smoothly instead of setting into little bits in the warm filling.
Save this sweet potato casserole with cinnamon and golden marshmallows for a cozy side dish with a soft, creamy center.
A watery filling starts before the casserole reaches the oven
Sweet potatoes hold onto water after boiling, and that extra liquid has nowhere to go once the filling is spread in the baking dish. If the mash feels loose or looks glossy with water, the finished casserole can bake up soft and wet rather than creamy. Draining well is the small, unglamorous part that makes the texture work.
After the cubes are fork-tender, drain them thoroughly before returning them to the pot. Let the potatoes sit in the colander briefly if water is still dripping, then mash them while warm with the butter, brown sugar, milk, cinnamon, and salt. The mixture should look cohesive and thick enough to hold a soft mound on a spoon. It doesn’t need to be stiff; the egg and first bake help it set into a scoopable casserole.
This also explains why I don’t add extra milk just because the mash seems thick at first. The sweet potatoes soften as you work in the butter and milk, and a filling that starts out loose is harder to correct once everything is mixed together.
What cinnamon, brown sugar, and sweet potatoes each bring
- Sweet potatoes — Fresh sweet potatoes give the filling its body and earthy sweetness. Choose firm ones without soft or damp spots, then peel and cut them into evenly sized cubes so they become tender at about the same time. Their moisture varies a little, which is another reason to drain them well.
- Light brown sugar — It sweetens the filling while adding a mild molasses note that suits the sweet potatoes. Pack it into the measuring cup so the amount stays consistent; there isn’t a need to buy a specialty brand.
- Whole milk and unsalted butter — Milk loosens the mash into a creamy filling, while butter adds richness and helps round out the sweetness. Whole milk gives a fuller texture than a thinner milk would, and unsalted butter leaves the salt level easy to control.
- Large egg — The egg helps the filling set during baking, so each spoonful holds together instead of slumping like loose mashed potatoes. Stir it in after the mixture has cooled for five minutes and mix until no streaks remain.
- Ground cinnamon — Cinnamon gives the sweet potatoes their warm, familiar aroma. Use a fresh jar if yours has been open for years; old ground cinnamon can smell faint and won’t contribute as much to the filling.
- Mini marshmallows — Their small size makes it easy to scatter an even layer across the casserole, and they toast quickly in the final bake. Mini marshmallows are the right fit here; large ones won’t cover the surface as evenly without cutting them up.
Boil, mash, and bake in two stages for a set center and toasted top
Cook the cubes until a fork slides in easily
Heat the oven to 350°F and grease a 9×13-inch baking dish. Put the peeled, 1-inch sweet potato cubes in a 6-quart pot with 8 cups of water, or enough to cover them, and bring the water to a boil over high heat. Reduce the heat and simmer for 15 to 20 minutes, until the cubes are fork-tender all the way through. If the fork meets a firm center, give them more time rather than trying to mash them partly cooked.
Mash while warm, then let the egg go in last
Drain the potatoes well and return them to the pot. Mash with the butter, brown sugar, milk, cinnamon, and salt until mostly smooth; a few soft bits are fine, but dry chunks will remain noticeable in the baked filling. Let the mixture cool for five minutes, then stir in the egg until fully combined. Scrape the sides and bottom of the pot as you mix so no streak of egg is left behind.
Bake the filling before adding the marshmallows
Spread the sweet potato mixture evenly in the prepared dish and bake for 20 minutes. An even layer gives the casserole a consistent bake, so smooth the corners rather than leaving a tall mound in the middle. The marshmallows aren’t added yet; putting them on at this point would leave them in the oven for the entire bake and give them too much time to brown.
Toast the topping and let the casserole settle
Scatter the mini marshmallows evenly over the casserole, then bake for 5 to 8 minutes, until they’re golden and the center reaches 160°F. Keep an eye on them near the end because the color can change quickly in those final minutes. Rest the casserole for 10 minutes before serving; the filling firms up slightly during that pause and is easier to scoop cleanly.
Two small changes that keep the casserole useful for more tables
Make the topping gluten-free
The sweet potato filling uses no wheat, barley, or rye. For a gluten-free table, choose mini marshmallows labeled gluten-free, since brands can differ in their ingredients and processing. The casserole keeps the same soft filling and toasted top.
Leave a little more texture in the mash
For a more rustic spoonful, stop mashing while a few small, tender pieces remain instead of working the filling fully smooth. The casserole will have a less uniform texture, but the cinnamon and brown sugar still blend throughout.
Storage and Reheating
- Refrigerator: Cover leftovers and refrigerate within 2 hours; keep them for 3 to 4 days. The marshmallow topping softens as it sits.
- Freezer: Freeze portions in an airtight container for up to 2 months. Thaw them in the refrigerator; the topping may be stickier after freezing, but the filling freezes well.
- Reheating: Warm covered portions in a 325°F oven until hot throughout, then uncover for the last few minutes if you want to dry the topping slightly. Reheat leftovers to 165°F, and avoid heating the whole dish repeatedly when you only need one serving.
Questions that come up when making sweet potato casserole
Sweet Potato Casserole with Marshmallows and Cinnamon
Ingredients
Equipment
Method
- Heat the oven to 350°F.
- Grease a 9x13-inch baking dish.
- Put the sweet potatoes and water in a 6-quart pot.
- Bring to a boil over high heat.
- Reduce the heat and simmer for 15 to 20 minutes, until the potatoes are fork-tender.
- Drain the potatoes well.
- Return the potatoes to the pot.
- Mash with the butter, brown sugar, milk, cinnamon, and salt until mostly smooth.
- Let the mixture cool for 5 minutes.
- Stir in the egg until fully combined.
- Spread the sweet potato mixture evenly in the prepared baking dish.
- Bake for 20 minutes.
- Scatter the marshmallows evenly over the casserole.
- Bake for 5 to 8 minutes, until they are golden and the center of the casserole reaches 160°F.
- Let the casserole rest for 10 minutes before serving.
