- 3 lb celery root (sometimes called celeriac; 2 large), peeled with a knife and cut into 1/2-inch pieces (8 cups)
- 1 3/4 lb boiling potatoes (3 large), peeled and cut into 1/2-inch pieces
- 1 cup heavy cream
- 4 oz black truffle butter or 1 stick (1/2 cup) unsalted butter
- 1 teaspoon salt
- 1/4 teaspoon black pepper
- Cover celery root and potatoes with cold salted water by 2 inches in a 4- to 6-quart heavy pot, then simmer, covered, until celery root is very tender, about 15 minutes.
- While vegetables are simmering, bring cream, truffle butter, salt, and pepper just to a simmer, stirring until butter is melted.
- Drain vegetables in a colander and transfer to a food processor. Add hot cream mixture in a steady stream, pulsing until smooth. (Alternatively, mash vegetables with hot cream mixture using a potato masher.)