Combine first 7 ingredients; beat at medium speed with an electric mixer until smooth. Pout batter into a greased 11 x 7 x 1 1.2 inch baking dish; set aside. Combine 3 tablespoons softened butter and remaining ingredients with a fork until well blended; sprinkle over potato mixture. Cover with marshmallows (if you want them). Baked at 350 for 35 minutes.