Preheat the oven to 400°F. Butter a baking sheet.

Boil until tender:

Drain the potatoes, return to the hot pan, and shake to dry them out. Rice the potatoes into a large bowl. Add while the potatoes are still hot:

Transfer the mixture to a pastry bag fitted with a large fluted or star tip and pipe onto the baking sheet in eight 4-inch-wide mounds. Bake until golden, about 20 minutes. Serve at once.