Take out ½ the mixture and place in a second bowl, sieve the cocoa powder into this mixture and mix until smooth.
Now, put a tablespoon of the vanilla mix in the middle of the tin. Then, using a clean tablespoon, put a blob of the chocolate mix in the middle of the vanilla one.
Keep doing this, alternating between vanilla and chocolate, each time you dollop a blob in, the whole mix will spread out on the base.
By the time you have used up both of the cake mixes, they should have just reached the edge of the tin. 2 ice-creams scoops are great for doing this if you have them.
Bake for 90 minutes, or until risen and firm to the touch.
Remove from the oven and leave to cool before transferring to a wire rack until completely cool.