Drop the potatoes into a large pot of cold salted water and bring to a boil.
Reduce heat and simmer, uncovered, until the potatoes are tender when pierced with a knife, about 25 minutes. Drain.
Put the butter and 3/4 cup buttermilk in the pot and add the drained potatoes. Place over very low heat and mash with a potato masher, adding more buttermilk as needed. Season well with salt and pepper.
PER SERVING: 145 calories, 3 g protein, 24 g carbohydrate, 4 g fat (3 g saturated), 12 mg cholesterol, 70 mg sodium, 2 g fiber.