Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender. Drain. In a large bowl, mash potatoes with cream cheese. Combine the egg whites, sour cream, onion powder, salt and peper; stir into potatoes until blended. Transfer to a greased 3 quart baking dish. Drizzle with butter. Cover and refrigerate overnight. Remove from the refrigerator 30 minutes before baking. Cover and bake at 350? for 50 minutes. Uncover and bake for 5-10 minutes longer or until a thermometer reads 160?.