4. Coat the bottom of an 8 inch pan with butter, add one layer of sweet potatoes and sprinkle half the mozzarella cheese and half of the cheddar cheese. Add a second layer of sweet potatoes and sprinkle the rest of the cheeses on top. Place in the oven and bake at 400 degrees until the cheese has melted and starts to bubble, about 15 minutes. Remove from the oven.