1. Peel potatoes, and cut into 1-inch pieces. Peel celery root using a paring knife, following the shape of the root. Cut into 1/2-inch pieces. Place potatoes and celery root in a small stockpot with enough water to cover, and bring to a boil over high heat. Reduce to a simmer, and cook until tender, about 10 minutes. Drain, return to pot; place over low heat to dry out.

2. Combine cream, butter, salt, and pepper in a small saucepan, and place over medium heat until butter is melted and mixture comes to a simmer. Pour over potato mixture, and combine, using a potato masher, until fluffy and smooth.