Black walnut layer cake

1/2 lb. butter
  2 c. sugar
  5 eggs, separated
  3 c. cake flour
  2 tbsp. cocoa
  1 c. buttermilk
  1/2 tsp. baking soda, stir in
     buttermilk
  2 c. walnut kernels, coarsely chopped
  1/2 tsp. vanilla or black walnut
     flavoring
  FILLING:
  2 sticks butter
  1 lb. confectioners' sugar
  1 tsp. vanilla flavoring
  Canned milk, if needed
     Save a tablespoon of nuts to garnish top of cake.  Cream butter
  with sugar.  Add egg yolks, beat.  Sift cocoa with cake flour and
  add alternately to butter mixture with buttermilk (mixed with soda).
   Add nuts and flavoring; mix thoroughly.  Beat egg whites until they
  hold peaks and fold in last.  Pour in 3 greased and floured cake
  pans. Bake at 350 degrees for about 25 minutes or until done. Soften
  2 sticks butter and beat in 1 pound confectioners' sugar, 1 teaspoon
  vanilla flavoring and (if needed) enough canned milk to make it of
  spreading consistency.  Mix well.  Spread between layers, on top and
  sides of cake.  Sprinkle a tablespoon chopped walnuts on top.