1 hr 10 min.
Preheat oven to 180°C | 350F | gas 4. Grease and line a 900 g | 2lb loaf 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 chis into the mixture in one of the bowls.
Spoon the chocolate and vanilla cake mixes into the tin alternately. Then take a skewer and swirl it around the mixture in the tin a few times to create a marbled effect.
Bake the cake for 45-55 mins until a skewer inserted into the centre comes out clean. Turn out onto a wire rack and leave to cool before serving.