Pre-heat oven to 425. Prepare the mashed potatoes according to the box recipe. When they are fully cooked, add the sour cream and the cream cheese. Mix very well. Add more potato flakes if the mixture seems runny. Pour into a greased 9x13 baking dish. Cut the butter into small pats, place on top of casserole. Bake for about 30 minutes. Allow to cool a bit before serving.