Instructions

1. Preheat the oven to 180C/350F/gas mark 4. Grease a 20cm cake tin and line the bottom with greaseproof paper.
2. Beat the butter and sugar together until light and fluffy. Add the eggs, one at at a time, mixing well. Sift the flour in and fold through with the milk and vanilla extract until well combined.
3. Divide the mixture into two bowls. Mix the cocoa powder with the warm water until it forms a paste and add to one of the bowls, stirring to combine thoroughly. Fold the chocolate chips through the chocolate mixture.
4. Take two spoons and dot dollops of the chocolate and vanilla cake mixes around the tin. When all the mixture is in the tin, tap it on the kitchen counter to make sure that there are no air bubbles, take a skewer and swirl it through the mixture a few times to create the marbled effect.
5. Bake the cake for 45-50 minutes, or until a skewer inserted into the middle of the cake comes out clean. Turn onto a wire rack and leave to cool before drizzling with ribbons of melted chocolate.