Potato Layer: Peel and slice potatoes about 1/4 inch thick. Cook in boiling water for about 15 minutes, or until potatoes are fork tender. Whip potatoes with electric mixer until moderately smooth - a few lumps are okay. Heat to warm 1/2 cup of the milk and add to potatoes, along with 1/2 cup butter and the sour cream. Add salt and pepper to taste. Whip til mixed, adding a little more milk, if needed, for the desired consistency.