Heat the Guinness in a saucepan, adding the butter in slices. Heat until the butter is melted then whisk in the cocoa and sugar. Beat the eggs with the sour cream, add the vanilla, then pour into the pan and mix well.
Add the flour and bicarbonate to the pan and mix thoroughly. Pour the mixture into your tin and bake for about 40 minute. A knife should come out clean when ready.
Leave the cake to cool and prepare the icing. Whip the cream cheese and sieve in the icing sugar. Beat the mixture and finally add the cream, whisk again until it thickens into a spreadable consistency. Ice the cake & the end result should look like the foam on top of a pint of Guinness!