Ingredients

• 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

Instructions

1. 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.
2. While vegetables are simmering, bring cream, truffle butter, salt, and pepper just to a simmer, stirring until butter is melted.
3. 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.)