Cut baked potatoes in half lengthwise, and scoop out pulp, leaving shells intact; set shells aside. Mash pulp; stir in butter and milk, and set aside.
Stir Parmesan cheese and next 4 ingredients into Bechamel Sauce; spoon evenly into shells. Top with mashed potatoes, and place on a baking sheet. Sprinkle with Cheddar cheese.
Bake at 325° for 15 to 20 minutes or until thoroughly heated.