Pre-heat the oven to 180°C / Gas Mark 4.
Grease a 20cm cake tin and line the bottom with a circle of greaseproof paper.
Beat the butter and WHITWORTHS DEMERARA SUGAR together, then add the eggs, one at a time, mixing well after each addition. Fold through the flour, milk and vanilla extract until the mixture is smooth.
Divide the mixture between 2 bowls. Stir the cocoa powder into one of the bowls. Take 2 spoons and use them to dollop the chocolate and vanilla cake mixes into the tin alternately, until all the mixture has been used up.
Take a skewer and use it to swirl the mixture a few times to create a marbled effect.
Bake for 50-60 minutes until a skewer inserted into the centre comes out clean.
If the cake isn’t cooked after 1 hour, cover loosely with a piece of foil and return to the oven for 10 minutes, or until cooked through.
Turn out onto a cooling rack and leave to cool.
To make the ganache topping, heat the cream in a pan and bring up to the boil, take off the heat. Place the chocolate in a large bowl, pour over the cream and stir to melt.
Allow to cool slightly, the mixture will thicken.
Transfer the cooled cake to a serving plate and pour the ganache over the top of the cake.
Drizzle with the melted white chocolate to decorate.