Preheat the oven to 350°F / 180°C. Butter a 9in / 23cm springform pan and line it with parchment paper.
Warm 250ml of stout in a large saucepan and melt 250g of unsalted butter in it. Whisk in 75g of cocoa powder and 400g of superfine sugar.
Beat 150ml of sour cream with 2 eggs and 1 tablespoon of vanilla extract and stir into the pan. Whisk in 275g of all-purpose flour and 2½ teaspoons of baking soda.
Pour into the springform pan and bake for 45-60 minutes, until a skewer comes out clean. Cool completely in the pan on a wire rack.
For the topping, beat 300g of cream cheese until smooth. Sift in 150g of icing sugar and 2 teaspoons of cornstarch and beat in, then beat in 125ml of heavy cream until spreadable.
Unmold the cake onto a plate and spread the topping over the top only.
