Preheat oven to 350 degrees and butter and line a 9" springform pan. Pour Guinness into a large, wide saucepan; add butter in spoons or slices and heat until the butter melts, at this time you should whisk in the cocoa and sugar.
Beat the sour cream with the eggs and vanilla and pour in the brown, buttery, beer mixture and finally whisk in the flour and baking soda. Pour cake batter into the greased pan and bake for 45 min. to 1 hour.
Leave to cool completely on a rack, cake will be quite damp. When cake is cold, sit on a flat platter or stand to frost.
Lightly whip cream cheese until smooth, sift in confectioners sugar and then beat them. Add the heavy cream and beat until it is of a spreadable consistency. Ice the top of the cake to resemble a pint of Guinness.