Preheat the oven to 160 fan. Grease and line a 33cm/22cm brownie tin.

In a large bowl, combine the flour, cocoa powder, sugars, bicarb and salt. In another bowl mix the sour cream and eggs together. Put the chocolate and butter in a heatproof bowl together melt in short bursts in the microwave. Stir together well. Pour the melted chocolate and butter into the flour mix, along with the sour cream and eggs and the hot water. Use a whisk to gently mix everything together to make a smooth, rich, chocolatey cake batter. Pour the batter into the tin and bake at 160 fan for 40 - 45 minutes, or until a skewer comes out clean.

To make the custard, put the milk, cream and half the sugar in a saucepan. Warm them together, gently on a low heat. In a bowl, beat together the egg yolks, the rest of the sugar, cornflour and cocoa powder. It will feel quite solid and grainy to start but keep mixing and it will eventually loosen up. When the milk mix is just steaming, remove it from the heat and pour it over the egg yolk mix, whisking to combine. Pour this back into the pan and return to a medium heat. Stir it constantly as it cooks gently, it will start to thicken and become shiny. Once it starts to gently simmer, cook it for 2 - 3 minutes, then taste the custard to check it doesn't have any powdery taste to it. Keep cooking for a couple more minutes if it does, if not, remove from the heat and stir in the chopped chocolate and salt. Cover the surface of the custard with clingfilm if you are not using it straight away.

To serve, cut the cake into 12 squares, dish them up and pour liberal amounts of hot chocolate custard on top.