Preheat the oven to 170 deg fan assisted. Grease and line a 10inch (25cm) x 9inch (23cm) baking tray with baking parchment.
Beat together the butter and sugar until light and fluffy and paler in colour.
225 g Butter softened, 225 g soft brown sugar
Add the beaten eggs a little at a time adding a tablespoon of flour as you go along.
4 medium eggs lightly beaten
Sieve together the remaining flour and baking powder and add to the butter sugar mixture along with the vanilla extract and 2 of the tablespoons of milk. Mix until well combined. Note: Leave the remaining tablespoon of milk for later.
275 g self raising flour, ½ teaspoon baking powder, 3 tablespoons milk, 1 teaspoon vanilla extract
Spoon half the vanilla cake mixture into the prepared baking tin in dollops.
Add the cocoa powder and the remaining tablespoon of milk to the remaining mixture that is left in the mixing bowl. Mix until well combined. Spoon the cocoa cake mixture in-between the vanilla cake batter.
1 ½ tablespoons cocoa powder
Using a cake skewer create swirls between the two flavours.
Bake in the oven for 35 - 40 mins or until baked, testing the centre of the cake with a skewer until it comes out clean. Leave in the tin for a few minutes to cool before turning out onto a cake rack to cool completely.
Melt the white and dark chocolate separately in heatproof bowls over a pan of simmering water. Place in separate piping bags ready to drizzle over the Marble cake.
50 g dark chocolate, 50 g white chocolate
When the cake has cooled completely drizzle the melted chocolate over the cake in a zig zag pattern. Leave the chocolate to set or place the cake in the fridge for 10 mins to speed up the process.