Preheat the oven to 180°C / 350°F / Gas mark 4.
Grease a 20cm cake tin and line the bottom with a circle of greaseproof paper.
Simply put all the ingredients (except the cocoa powder) into a food processor and blitz for 1-2 minutes until smooth.
Divide the mixture between 2 bowls. Stir the cocoa powder into the mixture in one of the bowls.
Use 2 spoons to dollop the chocolate and vanilla cake mixes into the tin alternately.
When all the mixture has been used up (and if you have helpers you’ll need to ensure the base of the tin is fairly evenly covered), tap the bottom to make sure that there aren’t any air bubbles.
Take a knife and swirl it around the mixture few times to create a marbled effect.
Bake the cake for 45-55 minutes until a skewer inserted into the centre comes out clean.
Turn out onto a cooling rack and leave to cool.
Freeze slices for up to 3 months then thaw overnight in the fridge and allow to come up to room temperature.