Preheat your oven to 200º C/390º F. Grease and flour a 22cm / 9-inch cake pan.
Melt the butter.
Whip egg whites.
In another bowl, cream butter and sugar. Add the egg yolks and mix. Sift in flour, baking powder and salt, and mix.
Fold in the egg whites and combine gently.
Divide the batter in two. Fold in the sifted cocoa powder to half of the batter. Alternately spoon vanilla and chocolate layers into the pan.
Bake for 50 minutes, or until a toothpick inserted in the center comes out clean.