5 medium Yukon Gold potatoes, unpeeled, cut into quarters
1 large celery root, peeled of tough skin and cut into 1-inch pieces
1 vegetable or chicken bouillon cube
2 tablespoons unsalted butter
3/4 cup to 1 cup sour cream
Kosher salt to taste
Black pepper to taste
Make-ahead tip: These potatoes can be made a day in advance; mix in 1/4 to 1/2 cup milk and reheat in the microwave for 2 minutes. Stir, then microwave an additional minute until hot.
In large pot, combine potatoes, celery root, bouillon cube with enough cold water to cover by two inches. Add tablespoon of kosher salt. Bring to boil, then reduce to simmer; allow to cook until the potatoes and celery root are tender, about 18 minutes.
Drain completely, return potatoes and celery root to the pot and add the butter. Using the back of a large fork or wooden spoon, smash the potato mixture against the pot until most of the lumps are gone.
Add 3/4 cup of the sour cream and mix well. If you desire a creamier mixture, add the additional 1/4 cup sour cream. Season with salt and pepper and serve hot.