In a large bowl, beat the mashed potatoes, milk, butter, salt and pepper until smooth. Stir in 1-1/3 cups cheese. Grease two 6-portion muffin pans; divide potato mixture between pans. Sprinkle with remaining cheese; top with parsley.
Cover and freeze one pan for up to 3 months. Bake the remaining pan, uncovered, at 350° for 15-20 minutes or until heated through and cheese is melted.
To use frozen potato cups: Thaw in the refrigerator. Bake as directed. Yield: 2 pans (6 servings each).