Peel off the potatoes and cut them into large cubes. Transfer the potatoes to a saucepan and cover with water. Boil the potato chunks until perfectly tender.
Remove the water and mash the potatoes in the same saucepan until smooth enough. Add in the butter, brown sugar, and seasoning (cinnamon, nutmeg, and salt) and mix to combine everything.
Grease a baking dish with non-stick cooking spray, or lightly brush with butter. Place in the potato filling spreading it evenly in the dish.
Add a layer of mini marshmallows and place the dish in a preheated oven at 350 degrees F.
Bake for 25 to 35 minutes or until the marshmallow layer softens a bit and turns golden brown. Enjoy!