Peel and quarter potatoes and in a large saucepan cover with salted cold water by 2 inches. Bring water to a boil and simmer potatoes until tender, about 15 minutes.
While potatoes are simmering, in a small saucepan heat butter with milk over moderately low heat until melted and keep warm.
Drain potatoes in a colander and force through a ricer or food mill into a bowl. With an electric mixer beat in milk mixture, yolks, and salt and pepper to taste.
Preheat oven to 400F. Spread about one third potatoes in a buttered 1-quart gratin dish and transfer remaining potatoes to a pastry bag fitted with a 3/4-inch star tip. On potatoes in dish pipe potatoes in the bag. Potatoes may be prepared up to this point 1 day ahead and chilled, covered loosely. Bake potatoes until heated through and tops of "ghosts" are golden, 15 to 20 minutes.
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