In my early twenties I worked as a pastrychef in an old-fashioned London Deli and Grocer. The cook was a lovely Irish woman who shared many recipes with me - my favourite being this one. We used to trim off the edges of the sheet cake to cut the slices into perfect squares - after we took the slices into the cafe, we would rush back downstairs to eat the slightly-warm-buttery-crumbled-messy cake ends, spilling cream cheese icing and dark, rich, moist crumbs all over ourselves in the process.
Preheat the oven to 180ºC. Grease a 23cm cake tin.
Put the Guinness and butter in a small saucepan and melt over a low heat – don’t let it boil, just warm until the butter melts.
Leaving on a very low heat, add the sugar and cocoa to the pot and whisk until smooth, add the sour cream and whisk again until smooth. Add the eggs and vanilla immediately, whisk until incorporated.
Add the sifted bicarb, then add the flour a small amount at a time, continuing to whisk. The mix will thicken, but you need to keep mixing for 5 minutes to make sure there are no lumps.
Pour the mixture into the tin and bake for 40 minutes or until a cake tester comes out clean.
Let cool, then ice with a simple vanilla buttercream or cream cheese icing.