In a medium pot, add potatoes, a teaspoon of salt and just enough water to cover the potatoes. Bring to a boil, then reduce heat to simmer until the potatoes are tender when pierced with a fork, about 45 minutes. Drain.
Return potatoes to the pan and set over very low heat, uncovered, for a few minutes, stirring occasionally, to let the potatoes dry out a little (too much moisture will dilute their flavor). Mash with a potato masher or fork. Beat the buttermilk into the potatoes with a wooden spoon until thoroughly incorporated. Beat in the butter. Add salt and pepper to taste.