Preheat oven to 180 degrees C. Grease and flour one 25cm ring tin.
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 1/2 of the batter and pour the rest into the prepared tin.
Mix cocoa into the remaining 1/2 of the batter. Then turn into ring tin 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 centre of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.