Quarter the potatoes and cover in the pot with cold water. Bring to a boil and cook for 15 minutes
Rice the potatoes and add the mascarpone and sour cream. Rice again. Add the butter and 1/3 of a cup of milk little by little until it starts to smooth out.
Add the horseradish, one teaspoon at a time.
Once it's fully incorporated, whip the potatoes with the remaining milk.