Place the potatoes in a saucepan with 3 quarts of cold water and 1 teaspoon of the salt. Bring to a boil and cook until tender, about 15 minutes. Drain well and return to saucepan; mash. Heat the butter and buttermilk in a small saucepan until hot. Pour over the potatoes and stir well. Season with the pepper, nutmeg, and the remaining salt.