3 pounds russet (baking) potatoes (6 to 8 medium)
1 medium head garlic
3/4 stick (6 tablespoons) unsalted butter, softened
1 teaspoon salt
1/4 teaspoon black pepper
Preheat oven to 350° F. Prick each potato once or twice with a fork. Cut off and discard top fourth of garlic head, then wrap garlic tightly in foil. Bake garlic and potatoes on same rack in lower third of oven until potatoes are tender, 50 minutes to 1 hour.
Remove potatoes from oven and cool on a metal rack 15 minutes. Continue to bake garlic until tender, about 15 minutes more, then cool in foil on rack. While garlic cools, halve potatoes lengthwise, then quarter each half (to form short wedges).
Scoop out potato flesh (reserving it for another use), leaving 1/4-inch-thick potato skins. Increase oven temperature to 425°F. Squeeze garlic into a small bowl, discarding garlic skins, and mash to a paste with butter, salt, and pepper using a fork.
Divide garlic paste among potato skins (about 1/2 teaspoon each), spreading evenly, then roast skins in a large shallow baking pan (1 inch deep) until golden and crisp, 20 to 25 minutes.