Sift together the flour, baking powder, and salt. Cream butter and gradually add sugar, creaming well. Add egg yolks, milk and melted unsweetened chocolate. Blend in the dry ingredients;mix thoroughly. Stir in nuts. Fold in 2 stiffly beaten egg whites. Turn into 9 x 9 inch pan, well greased and lightly floured on bottom. Bake in moderate oven (350 degrees) 25 to 30 minutes. Cool and frost with Chocolate Frosting. Cut into bars or squares. *For use with Pillsbury's Best Self-Rising Flour, omit baking powder and salt. FOR CUPCAKE BROWNIES: Fill muffin pans, lined with paper baking cups, 1/4 to 1/3 full. Bake 15 to 20 minutes. Cool and frost.