Preheat oven to Gas Mark 4, 180°C, fan 160°C. Butter a madeleine tray or a 12 hole muffin tin.
In a large bowl beat the butter and sugar together until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Fold in the flour, milk and vanilla extract until the mixture is smooth.
Divide the mixture between 2 bowls, and stir the cocoa powder and chocolate chips into the mixture in one of the bowls.
Spoon the chocolate and vanilla cake mixes into the moulds alternately. Then take a skewer and swirl it around the mixture in the moulds a few times to create a marbled effect.
Bake the cakes for 7-8 mins until a skewer inserted into the centre comes out clean. Turn out onto a wire rack and leave to cool before serving.