Cream one-half cupful of butter or oleo until smooth and beat in one cupful of sugar. Add the yolks of three eggs well beaten and one teaspoonful of vanilla. Sift together one and one-half cupfuls of flour, six tablespoonfuls of cocoa, two teaspoonfuls of baking powder and a pinch of salt. Add two-thirds of a cupful of milk and the sifted ingredients alternately, beating very thoroughly. Fold in the stiffly beaten egg whites and pour into well greased pans and bake in a moderately hot oven for about thirty minutes.