Mix 225g butter, 225g caster sugar, 4 eggs, 225g self-raising flour, 3 tbsp milk and 1 tsp vanilla extract into a food processor and whizz for 1-2 mins until smooth.
Divide the mixture between 2 bowls. Stir 2 tbsp cocoa powder into the mixture in one of the bowls. Take 2 spoons and use them to dollop the chocolate and vanilla cake mixes into the tin alternately.
Take a skewer and swirl it around the mixture in the tin a few times to create a marbled effect.
Bake the cake for 45-55 mins until a skewer inserted into the centre comes out clean. Turn out onto a cooling rack and leave to cool.
