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Classic Sweet Potato Casserole with Pecan Streusel and Marshmallows

Classic sweet potato casserole with pecan streusel and marshmallows—silky, buttery mashed sweet potatoes baked under a crunchy brown sugar pecan crumble and finished with toasted mini marshmallows. The result is creamy filling with bubbling edges, crisp streusel, and gooey golden topping.
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Prep Time 25 minutes
Cook Time 48 minutes
resting 10 minutes
Total Time 1 hour 23 minutes
Servings: 8 servings
Course: Side Dish
Cuisine: American
Calories: 420
Ingredients Equipment Method Notes

Ingredients
  

sweet potato filling
  • 3 lb sweet potatoes peeled and cut into 1-inch cubes
  • 1 tsp salt for the cooking water
  • 4 tbsp unsalted butter melted
  • 0.3333333333 cup granulated sugar
  • 0.5 cup whole milk
  • 2 large eggs
  • 1 tsp vanilla extract
  • 0.5 tsp ground cinnamon
  • 0.25 tsp ground nutmeg
  • 0.5 tsp salt for the mash
pecan streusel
  • 0.75 cup packed light brown sugar
  • 0.3333333333 cup all-purpose flour
  • 0.25 tsp ground cinnamon
  • 4 tbsp unsalted butter melted
  • 1 cup pecans roughly chopped
topping
  • 2 cup mini marshmallows

Equipment

  • 1 sheet pan
  • 1 Dutch oven

Method
 

Cook and mash the sweet potatoes
  1. Place the sweet potato cubes in a large pot, cover with cold water, and add 1 tsp salt. Bring to a boil, then simmer for 15–18 minutes, until a fork slides easily into the potatoes.
  2. Drain the potatoes well and steam-dry in the colander for 5 minutes. This helps keep the mash silky instead of watery (visual cue: no visible steam pooling in the colander).
  3. Mash the sweet potatoes in a large bowl until smooth. Make sure no large lumps remain for a creamy texture.
  4. Stir in the melted butter, granulated sugar, whole milk, vanilla extract, ground cinnamon, ground nutmeg, and ½ tsp salt until creamy. Beat in the eggs until fully combined.
Build the casserole
  1. Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish. Spread the filling evenly in the prepared dish.
  2. For the streusel, mix the brown sugar, all-purpose flour, and ground cinnamon in a bowl. Stir in the melted butter until clumpy crumbs form (visual cue: it should look like damp crumble).
  3. Fold the pecans into the clumpy streusel mixture. Distribute evenly so the topping bakes with crunch.
  4. Sprinkle the streusel evenly over the sweet potatoes. Keep coverage even so each bite has crunchy topping.
Bake and toast the marshmallows
  1. Bake at 350°F (175°C) for 25 minutes, until the edges are bubbling and the streusel is fragrant. Visual cue: bubbling around the perimeter and set, golden-brown crumbs.
  2. Scatter the mini marshmallows over the top. Bake for another 8–10 minutes until they’re puffed and toasted golden brown (visual cue: browned peaks and glossy melted edges).
  3. Let the casserole rest for 10 minutes before serving warm. This sets the filling so it slices and spoon-serves cleanly.

Notes

For the smoothest texture, steam-dry the cooked sweet potatoes for the full 5 minutes before mashing. Cover and refrigerate leftovers for up to 3 days; reheat in the oven at 325°F (163°C) until warmed through. Freezing is not recommended because the marshmallows and streusel can lose texture after thawing. For a lighter version, swap whole milk for unsweetened oat milk and use plant-based butter in equal amounts.