Preheat the oven to gas mark 4/180°C/160°C Fan/350ºF, and butter and line a 23cm / 9 inch springform 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 bicarb.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 cornflour and then beat to combine.If using double cream, add it and beat until you have a spreadable consistency. If using whipping cream, whisk first to soft peaks, add a couple of spoonfuls into the cream cheese mixture and once this is combined, fold in the rest.Ice the top of the black cake so that it resembles the frothy top of the famous pint.