In mixing bowl, cream butter and 1 cup of sugar. Add egg yolks; beat well. Add potatoes and mix thoroughly. Combine flour, cocoa, baking powder, cinnamon, salt, nutmeg, and cloves. Add dry ingredients to creamed mixture alternately with milk, beating until smooth. Stir in vanilla and nuts. Beat egg whites until foamy; fold into batter. Pour into greased and floured 13 x 9 x 2-inch baking pan. Bake at 350 degrees for 40 to 45 minutes. Cool. Serves 16 to 20.