Put the potatoes in a medium pot, cover with cold water and add 2 tablespoons salt. Bring to a low simmer and cook the potatoes, uncovered, until tender, about 45 minutes. Drain.
Peel the potatoes and return them to the pot (for smoother mashed potatoes pass them through a ricer or food mill after peeling). Mash the potatoes with the buttermilk and butter using a fork or potato masher. Stir in 2 teaspoons salt and a few grinds of pepper. Sprinkle with the scallions.
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