In a large saucepan, cover the potatoes with cold water and add the salt. Boil over high heat until tender, 15 to 18 minutes. Drain, reserving 1/3 cup of the cooking liquid. Pass the potatoes through a ricer or food mill set over the pan. Stir in the buttermilk, butter and chives. Season with pepper. Add the reserved water to desired consistency and rewarm over moderately low heat before serving.