grease and line a 20 cm cake tin, Preheat Oven to 180°C
Mix the Butter, Caster sugar, eggs, Self raising flour , Milk and vanilla extract together in a Food Processor
If not using a food processor, beat the butter and sugar together, then add the eggs, one at a time, mixing well after adding each one
Then fold through the flour, the milk and the vanilla extract until smooth
Divide the batter between two bowls. stir the Cocoa Powder into one of the bowls
Alternating between the two mixes, one at a time, spoon the mixtures into the Cake tin, making sure to cover the bottom of the tin, make sure to tap the bottom of the tin on your work surface to ensure no air bubbles
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 cooling rack and leave to cool