Place the celeriac cubes into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer 12 minutes. Add the potatoes, and continue boiling until the vegetables are very tender, about 15 minutes more. Drain and allow to steam dry for a minute or two.
Return the vegetables to the pot, and stir over medium-high heat until liquid is no longer pooling from the vegetables. Remove from the heat, and pour in the cream and butter. Mash with a potato masher until almost smooth.