Place potatoes and celery root in a pot and cover with cool water. Bring to a boil over medium high heat. Cook, covered, for 20 to 25 minutes. Reserve one cup cooking water; drain the potatoes.

Return the pot as-is over the fire and reduce heat to low. Add vegan butter and stir until melted. Add the garlic, salt, and truffle salt and mix well.

Return the chopped potato mixture and add half a cup reserved water. Mash with a potato masher, add another quarter cup cooking water, and mash until smooth.

To serve, spoon onto plate and sprinkle more truffle salt. Serve hot.