Chocolate Guinness Cake

For the cake:
250 ml Guinness
250 g unsalted butter
75 g cocoa
400 g caster sugar
142 ml sour cream
2 eggs
1 tbsp real vanilla extract
275 g plain flour
2 1/2 tsp bicarbonate of soda
For the icing:
300 g Philadelphia cream cheese -
150 g icing sugar
125 ml double or whipping cream
Instructions:

Preheat the oven to gas mark 4 / 180oc, butter and line a 23 cm springform tin
Pour the Guinness into a large wide saucepan, add the butter gradually, and heat until the butter has melted. At which time, whisk in the cocoa and sugar.
Beat the sour cream with the eggs and vanilla and pour into the brown, buttery, beery pan and finally whisk in the bicarb and flour.
Pour the cake batter into the greased, 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.
Lightly whip the cream cheese until smooth, sieve over the icing sugar, then beat together. 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 a pint.