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.