Cover potatoes with salted cold water by 1 inch in a saucepan and simmer until tender, about 20 minutes. While potatoes are simmering, cook shallots in butter in a small nonstick skillet over moderately high heat, stirring frequently, until softened and deep golden, about 8 minutes. Drain potatoes, return to pot, and coarsely mash with a potato masher. Stir shallots into potatoes with buttermilk and salt and pepper to taste. Each serving about 193 calories and 2 grams fat