Preheat the oven to 355°F, and grease and line a 9″ springform pan.
Pour the Guinness into a large saucepan, and add the sliced butter. Heat until the butter is melted, and remove the saucepan from the heat. Whisk in the cocoa and sugar.
Beat the sour cream with the eggs and vanilla in a bowl or mixer; then pour the chocolate mixture into the sour cream mixture and blend.
Finally beat in the flour and baking soda.
Pour the batter into a the springform pan, bake for 50-60 minutes (touch the top of the cake to test, it will spring back when touched if done). Leave to cool completely in the pan.
For the icing: Beat the powdered sugar into the cream cheese. Add the cream, and beat again until it’s spreadable. Ice the top of the cake only (so it looks like the frothy top of a glass of Guinness).