Beat icing sugar and butter till light and fluffy. Add egg yolks and vanilla and beat till smooth.
Beat the egg whites with a clean whisk, then add to the butter mixture. Gently fold in the sieved flour and baking powder.
Separate half of the mixture, placing in another bowl. Add the cocoa to one mixture, folding in till well incorporated.
Grease a sandwich tin with butter and dust with flour. Add the vanilla cake mixture, then the cocoa mixture. Use a knife and swirl it through the mixtures to create a marbled effect.
Bake cake for 20 minutes, or till a skewer inserted in the centre comes out clean.