Put the potatoes in a large saucepan, cover with cold water by about 1 inch and add 2 tablespoons salt. Bring to a boil, uncovered, over high heat, and then reduce the heat to medium-high and simmer briskly until fork tender, about 45 minutes. Drain the potatoes, reserving 1/2 cup of the water. Quarter the potatoes and pass them through a food mill or ricer back into the pan. (Peel them first if you are using a ricer).
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.