Heat milk with butter over moderate heat in a small saucepan just until hot (do not boil). Carefully remove potatoes from foil and let cool slightly, about 10 minutes. Halve potatoes lengthwise and scoop out into a bowl
, reserving skins of 8 halves with a 1/4-inch border of potato around skin. Mash potatoes with milk mixture until almost smooth. Stir in sour cream, chopped chives, and 1/4 cup
cheese; salt and pepper to taste. Mound mixture into reserved potato skins and sprinkle with remaining 1/4 cup cheese.