Preheat the oven to 180°C (350°F) (gas 4).
Break up the chocolate and put it in a heatproof bowl with half the milk. Sit the bowl over a pan of just simmering water to melt the chocolate, about 5 minutes. Make sure the base of the bowl does not touch the water. Stir the chocolate and milk until combined and then put to one side to cool.
Now in a small bowl, lightly whisk together the remaining milk, crème fraîche (or sour cream), and brandy. Put to one side.
Next, using an electric mixer, cream the butter and sugar together until soft and fluffy, about 2 minutes.
Now gradually add the eggs until the mixture is smooth. Occasionally scrape down the sides of the bowl to incorporate all the mixture.
Using a large metal spoon fold in half the flour with a pinch of salt and then add half the milk mixture. Repeat using the remaining flour and milk mixture.
Spoon slightly less than half the cake mixture into a separate bowl and quickly mix in the melted chocolate.
Then combine both batches of the cake, marbling lightly so that both mixtures are still apparent.
Carefully spoon into the prepared cake pan and bake for 40 minutes, until a fine skewer comes out clean when poked through the centre of the cake.
Leave in the pan until the cake is fairly cool and then turn out on to a wire rack.
Dust the top with icing/powdered sugar to serve.