In a large saucepan combine the potatoes with enough salted water to cover them by 2 inches, bring the water to a boil, and simmer the potatoes for 1 hour, or until they are very tender. Drain the potatoes and let them cool until they can be handled. Peel the potatoes and force them through a ricer into a bowl.
In a small saucepan heat the butter and buttermilk over moderately low heat, stirring, until the butter is melted, pour the mixture over the potatoes, and add 1 teaspoon of the paprika and salt and pepper to taste. Stir the mixture until it is combined well, transfer it to a large pastry bag fitted with a large decorative tip, and pipe six 3-inch rosettes onto a well-buttered baking sheet. The potato rosettes may be prepared up to this point 1 hour in advance and kept at room temperature. Sprinkle the rosettes with the additional paprika, bake them in the upper third of a preheated 425°F. oven for 6 minutes, or until they are hot, and transfer them carefully with a metal spatula to a platter.