350g self-raising flour
2 ½ tbsp cocoa powder
2 tsp baking powder
300g caster sugar
4 large free-range eggs, beaten
300ml sunflower oil
1 tsp vanilla extract
Preheat the oven to 180ºC/350F/Gas Mark 4. Grease a deep 23cm/9″ round cake tin with a removable base and line with parchment paper. Sift the flour, and baking powder into a large bowl. Add the sugar and mix well.
Place the wet ingredients into a measuring jug and whisk together. Make a well in the center of the dry ingredients and pour in the contents of the jug.
Beat together by hand or with an electric hand-mixer until you have a smooth mixture. 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.