Melt butter in a large skillet over medium heat. Add sweet potatoes and cook, stirring frequently, until tender, about 20 minutes. Remove from heat and stir in flour, baking powder, cinnamon, nutmeg, and salt. Stir in milk and milk mixture. Pour into prepared casserole dish. Bake in preheated oven at 350? for 40 to 45 minutes, or until top is brown and bubbly. Let stand 5 minutes before serving.