Bring a large pot of salted water to a simmer (1 tablespoon of salt for every 2 quarts of water) while you peel the celery root with a knife. Cut into 1-inch pieces and boil for 25-30 minutes, until soft.
Drain celery root, returning it to the pot on low heat. Allow to steam for a minute or two, shaking the pan a bit to prevent sticking.
Add milk or cream, butter, and a generous pinch of salt. Mash with a potato masher until it is as smooth as you like it. Add salt to taste and garnish with the celery root leaves, if desired.