Preheat oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
In a large bowl, combine the mashed sweet potatoes, melted butter, sugar, milk, vanilla extract, cinnamon, nutmeg, and salt. Mix until well combined and smooth.
Spread the sweet potato mixture evenly into the prepared baking dish.
Bake for 20 minutes, or until heated through.
Remove from oven and top evenly with mini marshmallows.
Return to oven and bake for an additional 5-10 minutes, or until marshmallows are golden brown and puffed.
Serve warm.
Notes
For a smoother casserole, use an electric mixer to mash the sweet potatoes.
You can prepare the sweet potato mixture ahead of time and refrigerate. Add marshmallows and bake just before serving.
If you prefer a less sweet casserole, reduce the amount of granulated sugar.