Place the potatoes in a large pot and cover with cold water. Bring to a boil and add 2 1/2 tablespoons salt. Reduce heat and simmer until just tender, about 15 minutes. Drain the potatoes and return to the pot.
Add the butter, buttermilk, cream, nutmeg, 1/2 teaspoon salt, and 3/4 teaspoon pepper. Mash the potatoes with a handheld masher. Transfer to a serving dish.