Salting butter serves both taste and preservation purposes, since the salt slows down bacterial growth in the tiny water droplets trapped within the butter. The salt used for this purpose is ordinary food grade sodium chloride, the same table salt used in cooking. Calcium chloride is instead used in cheese making to help the milk coagulate, and potassium chloride only shows up in special low sodium butter variants, not in standard salted butter. So the salt used in regular salted butter is sodium chloride, NaCl.