You need a special danish aebleskive pan. It can be purchased in most scandinavian specialty stores, or be ordered through gourmet cookware stores.
Mix flour, salt and sugar. Beat together buttermilk and egg yolks and add to the flour mix. Add soda. Beat the egg whites until they are stiff and fold them into the dough. Heat the aebleskiver pan and put melted butter in each hole in the pan. Pour batter into the holes, but do not fill them completely. Put the pan over low heat and turn the aebleskiver when they are half done. Service with jelly or applesauce.