Duchesse Potato Ghosts

["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 400 degrees F.", "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 plain tip.", "On potatoes in dish pipe potatoes close together into pointed mounds to form ""ghosts"" and garnish each mound with 2 seeds for ghosts' ""eyes.""