Preheat oven to 375°F. Grease a 9x13-inch baking pan with butter or nonstick spray. Set aside.
Add a steamer basket to a pot filled with 2 inches of water. Add in the sweet potato, cover, and bring the pot up to a simmer. Once the steam is visible, cook for 10-15 minutes until the potatoes are easily pierced with a fork.
3 pounds sweet potatoes
Transfer the potatoes to a large bowl. Add in the butter, and mash and stir with a potato masher until smooth.
⅓ cup unsalted butter
Stir in ½ cup of the pecans, brown sugar, vanilla extract, cinnamon, salt, and nutmeg.
1 cup chopped toasted pecans, ½ cup brown sugar, ½ teaspoon pure vanilla extract, 1 teaspoon ground cinnamon, ½ teaspoon kosher salt, ⅛ teaspoon ground nutmeg
Spread the mixture into your prepared baking dish. Sprinkle on the marshmallows and chopped pecans.
2 cups mini marshmallows
Bake for 20-25 minutes until the marshmallows are melted and golden brown.