Peel, sliced potatoes and boil in a small amount of water until soft, then drain.
Combine mayonnaise, sour cream, 1 cup cheese and butter.
Add potatoes, salt and pepper and beat with an electric beater until creamy.
Pour into a lightly greased 1-quart baking dish.
Bake at 350Â° until bubbling around the edges, about 20 minutes.
Cover with 1/2 cup cheese and bake until cheese melts.