Preheat the oven to gas 4/180 degrees. Line muffin tin with 12 paper cases. In a bowl, beat the butter and sugar until light and fluffy.
Add the vanilla, then beat in the eggs, one by one, until smooth. Beat in the rest of the flour.
Put half the mixture in another bowl. Add the cocoa powder and melted milk chocolate to one bowl then add the milk to the other mixture.
Fill each case with spoonfuls of both mixtures then swirl a skewer through the mixtures to create a marble effect. Bake for 20 - 25 minutes.