Preheat oven to 375°. Pierce potatoes several times with a fork, and bake about 1 hour or until tender.
Cut a "lid" lengthwise off top of each potato, and scoop out flesh, making sure to leave skins whole. Pass flesh through a food mill or mash with a potato masher, and place in a large bowl. Stir in milk, butter, and salt.
Divide mixture evenly among potato skins, sprinkle with pepper, and bake at 375° for about 30 minutes or until lightly browned. Skins can be stuffed and refrigerated up to 1 day ahead of serving; increase second baking time to 45 minutes to 1 hour.