Preheat the oven to gas mark 4/180°C/160°C Fan/350ºF.
Butter a 23cm/9 inch springform tin.
Pour the Guinness into a large saucepan.
Add the butter and heat until the butter is melted.
Whisk in the cocoa and sugar.
Beat the sour cream with the eggs and vanilla extract and pour into the saucepan mixture.
Whisk in the flour and bicarbonate of soda.
Pour the cake batter into the greased tin.
Bake for 45 minutes to an hour.
Leave to cool completely in the tin on a cooling rack.
Ice the top of the black cake so that it resembles the frothy top of the famous pint.