I dearly love this recipe and have been making it for years, sometimes as a cake and sometimes as cupcakes (be warned: this makes a lot of cupcakes). The cake is chocolatey and rich, with a lovely note of Guinness that is there but not overpowering. It is never dry and keeps quite well. And the frosting! Let's just say that cream cheese in frosting is mostly always a good thing.
Preheat oven to 350℉. Line two cupcake tins with liners.
Pour the Guinness into a large saucepan, slice up the butter, and add to the beer. Heat over medium heat until the butter melts. Whisk in the cocoa and sugar, then set aside to cool briefly.
In a large bowl, beat the sour cream with the eggs and vanilla. Pour in the beer, then stir in the flour and baking sodauntil incorporated.
Divide the batter equally between the cupcake tins (I got about 20 cupcakes), place in the oven, and bake until risen and firm, about 25 minutes, and a tester inserted in a cupcake comes out clean. Remove from oven and let cool 5 minutes in the pan, then turn out onto a rack to cool completely.
Make Frosting
In a large bowl, whip the cream cheese with an electric mixer. Sift the powdered sugar over and beat to incorporate. Then beat in the heavy cream.
Frost each cupcake. Nigella says: "Ice the top of the black cake so that it resembles the frothy top of the famous pint."