Chocolate marble cake

1/3 c  butter
      2 ea Egg, well beaten
      1 c  Sugar
  1 1/2 c  flour
      2 t  baking powder
    1/2 c  Milk
      1 ea chocolate, unsweetened, squa
      1 T  butter
      1 t  vanilla
 
  Cream the 1/3 cup butter and sugar together, add the well beaten eggs
  and mix well. Sift flour and baking powder and add alternately with
  the milk to the first mixture. Put 1/3 of mixture into a bowl and add
  the 1 Tbsp of butter and chocolate which have been melted together. To
  the white batter, add the vanilla. Drop white batter, then chocolate,
  by spoonfuls into a well-greased, deep cake pan and bake at 350-F
  about 40 minutes.
  Source:  Pennsylvania Dutch Cook Book - Fine Old Recipes, Culinary
  Arts Press, 1936.