This is the country mash of your dreams. You don't even have to peel the potatoes before you start. Cooking them in their skins makes them more flavorful, and the skins slip off in a food mill.
When the potatoes are almost done, heat the buttermilk in a small saucepan over medium-high heat to just under a simmer. Set aside; it will separate and look curdled but don't worry, it will mix in fine.
Fold the buttermilk and reserved cooking water into the potatoes over low heat. Then fold in the butter a handful of pieces at a time until fully incorporated. Season with salt and pepper. Serve hot.