Peel and quarter potatoes. Combine with cold well salted water to cover by 1 inch in a 4 quart saucepan, bring to a simmer, and simmer until tender; about 20 minutes.
Meanwhile, melt butter in a small nonstick skillet over moderately high heat. Add shallots and cook, stirring frequently, until softened and deep golden, about 8 minutes.
Drain potatoes in a colander, return to pan and coarsely mash with a potatoes masher. Stir in shallots, buttermilk, a bit of butter, salt and pepper.