Cut baking potatoes lengthwise in half; scoop out insides, leaving a thin shell. Mash potatoes in medium bowl until no lumps remain. Add small amounts of milk, beating after each addition. (Amount of milk needed to make potatoes smooth and fluffy depends on kind of potatoes.) Add 1/3 cup butter, the salt and pepper; beat vigorously until potatoes are light and fluffy. Fill half of potato shells with mashed potatoes.