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.
