Made this cake for my husband's birthday - it was a hit! I can't say that you can absolutely taste the stout in it, but there is certainly a resonant, ferrous tang which we happen to love. There is enough sugar - a certain understatement here - to counter any potential bitterness of the Guinness. Made a cream cheese frosting to echo the pale head that sits on top of a glass of stout. It's unconventional to add cream but it makes it frothier and lighter which I regard as aesthetically and gastronomically delightful.
Preheat the oven to gas mark 4/180°C/350°F, and butter and line a 23 cm spring form tin.
Pour the Guinness into a large wide saucepan, add the butter - in spoons or slices - and heat until the butter's melted, at which time you should whisk in the cocoa and sugar. Beat the sour cream with the eggs and vanilla and then pour into the brown, buttery, beery pan and finally whisk in the flour and baking soda.
Pour the cake batter into the greased and lined tin and bake for 45 minutes to an hour. Leave to cool completely in the tin on a cooling rack, as it is quite a damp cake.
When the cake's cold, sit it on a flat platter or cake stand and get on with the icing. Lightly whip the cream cheese until smooth, sieve over the icing sugar and then beat them both together. Or do this in a processor, putting the unsieved icing sugar in first and blitz to remove lumps before adding the cheese.
Add the cream and beat again until it makes a spreadable consistency. Ice the top of the black cake so that it resembles the frothy top of the famous pint.