Preheat the oven to 180ºc/Gas 4. Grease an 8 inch deep cake tin.
Beat together the margarine and sugar until pale and fluffy.
In a separate bowl, lightly beat the eggs with the vanilla extract. In another large bowl add the flour.
Add a ¼ of the eggs to the butter and stir in, then fold in ¼ of the flour and stir until combined and then repeat until all the eggs and flour are mixed together and smooth.
Divide the mixture between 2 bowls then stir the cocoa into one of the bowls.
Spoon the cake mixes alternately into the tin. Take a skewer and swirl the mixtures a few times to create a marbled effect.
Bake for 40-45mins until a skewer inserted in the middle comes out clean. Turn out onto a wire rack and leave to cool.