Peel and cube the sweet potatoes, then boil in a large pot until tender, about 15-20 minutes. Drain and return to pot.
Mash the sweet potatoes until smooth, then mix in brown sugar, milk, butter, vanilla, cinnamon, nutmeg, and salt until well combined.
Prepare the marshmallow topping by measuring out 1 cup of mini marshmallows. Optionally mix in chopped pecans or walnuts.
Transfer the sweet potato mixture to a greased 9x13-inch baking dish and smooth the top.
Bake in a preheated oven at 350°F for 25 minutes. Remove from oven, sprinkle marshmallows on top, and bake for an additional 10-15 minutes until marshmallows are golden brown.
Let the casserole cool for a few minutes before serving.