How to Make It
Place the potatoes and 2½ tablespoons salt in a large pot. Cover with cold water. Bring to a boil. Reduce heat and simmer until tender, 20 to 25 minutes. Meanwhile, in a saucepan, over medium heat, heat the butter and ½ cup of the heavy cream until the butter melts.
Drain the potatoes and return to pot. Add the butter-cream mixture, sour cream, nutmeg, pepper, and the remaining salt. With a handheld potato masher, mash the potatoes. If a thinner consistency is desired, add the remaining cream, 1 tablespoon at a time.
Transfer the mashed potatoes to a heatproof serving dish.