Celery-Root Purée with Truffle Butter

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.)