Sift salt and baking powder with the flour; rub in the butter. Mix and add the beaten yolks and milk, and last stir in the whites which have been beaten to a stiff froth. Bake at once in well-greased waffle-irons. By using two cups of milk, the mixture is right for pancakes. If sour milk is used, substitute soda for the baking powder. Sour cream makes delicious waffles.