Place the potatoes in a saucepan and cover with cold water. Add salt and bring to a boil, then cook until the potatoes fall off a fork when pierced, about 20 minutes. Drain the potatoes, reserving about a 1/2 cup of the cooking liquid. Using a potato ricer or sieve and rubber spatula, gently press the potatoes over a large bowl. Mix in the butter, salt, buttermilk, and green onions and stir to blend. Add a tablespoon or 2 of cooking liquid if necessary to obtain desired consistency. Salt and pepper liberally and serve.