Beat the mashed potatoes with the sour cream, the shredded cheese, salt and pepper, and the egg yolks. A blender does the best job of this in about 2 minutes. Stir in the chopped chives or scallions.
Beat the egg whites with a little salt and a pinch of cream of tartar until stiff. Fold them carefully into the potato mixture and pile it into a buttered souffle dish. Bake in a preheated, 350° degree oven for about 45 minutes and serve at once.