Heat potatoes till hot and done; mash. Add butter, milk, brown sugar, nutmeg and salt. Place in a buttered round ovenproof dish, making a ring around the edge leaving a hole in the middle for the sauce.
For the sauce: Mix together 1/2 cup butter, 3/4 cup brown sugar, 2 tablespoons flour and dissolve on low heat. Add 1 cup water and cook until thickened. Spoon into middle of potatoes. Place mini marshmallows on top and brown under broiler in oven.