Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 10 inch tube pan.
In a large bowl, cream the butter with the sugar. Beat in egg yolks. Continue beating for 10 minutes.
Gradually stir in flour, alternating with splashes of milk, until all of the flour and milk are mixed in. Stir in baking powder.
In a separate bowl, beat egg whites. Fold egg whites into batter.
Reserve 1/4 of the batter, and pour the rest into a greased, floured tube pan.
Mix cocoa into the remaining 1/4 of the batter. Then turn into tube pan, and fold under with a fork to produce a marbled appearance.
Bake in the preheated oven for 60 to 70 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.