Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Line a muffin tin with Muffin Cases. To make the cheesecake mixture, beat the cheese in a bowl to soften. Add 25g (1oz) flour, 50g (2oz) sugar, 1 egg and the Vanilla Extract. Mix well and set aside.
For the chocolate mixture, break the Dark Chocolate into pieces and put in a heatproof bowl with the butter over a saucepan of barely simmering water until melted. Stir in the remaining sugar, remove from the water and leave to cool for 10 minutes.
Sift the remaining flour on top along with the Bicarbonate of Soda and add the remaining egg and buttermilk. Mix well.
Spoon about 10ml (2tsp) of the chocolate mixture into each case, then top with the same amount of cheesecake mixture. Divide the remaining chocolate mixture between the cases and gently swirl the 2 mixtures together. Spoon over the remaining cheesecake mixture – the case should be just over half full.
Bake in the oven for about 25 minutes until risen and just firm to the touch. Cool for 5 minutes then transfer to a wire rack to cool completely.