Special equipment: Ricer
In a large, heavy-bottomed pot add enough water to completely cover the potatoes. Bring the water to a boil over medium-high heat. Add and the potatoes and a large pinch of salt. When the water returns to a boil, reduce heat to a simmer, and cook until tender, about 30 minutes.
Meanwhile, put the diced celery root in a saucepan with enough water to cover, bring to a boil, loosely cover, and reduce the heat to a simmer. Cook until tender, about 30 minutes. It should be the same consistency as the cooked potatoes.
Drain the potatoes. Put them through a ricer and add 2 tablespoons of the butter and half the cream.
Drain the celery root and put it in a blender with the remaining 2 tablespoons of butter and remaining cream. *Puree until smooth, using care, as the puree will be very hot.
Add the celery root puree to the potatoes and mix well. Lightly season with salt and pepper. .
When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.