Celery-Root Purée with Truffle Butter

Serves 4

1.5lb Celery Root (sometimes called celeriac; 2 large), peeled with a knife and cut into 1/2-inch pieces (8 cups)
1 lb boiling Potatoes (3 large), peeled and cut into 1/2-inch pieces
1/2 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.)