Peel celery root, dice cauliflower, dice radishes; place into a large pan and cover in broth.
Cover and bring to a boil over high heat. Uncover, reduce heat to medium low, and cook at a lively simmer until cauliflower can be easily pierced with a sharp knife and celery root starts to break up, about 17 to 20 minutes.
Drain mixture through a colander, reserving liquid. Transfer to the bowl of a food processor fitted with a blade attachment. Add butter, sour cream, parmesan, truffle powder, salt, and pepper
Process to a smooth purée; adding reserved liquid as needed, stopping to scrape down the sides of the bowl as necessary, about 3 minutes from start to finish.
Taste and season with white pepper as needed.
Serve immediately or reheat gently in a saucepan with some heavy cream to prevent it from drying. You can also reheat it in a double boiler.