Cake: Preheat oven to 180. Grease 2 round cake tins and dust with flour.
Heat Guinesss, butter, chocolate and sugar in a water bath over low heat until the chocolate has melted.
Allow to cool but not so much that the chocolate sets.
Mix the eggs and sour cream into the chocolate mixture. Mix the flour and bicarbonate soda then fold into the batter. Stir well to avoid forming lumps.
Pour half of dough into each prepared cake tin and bake in the preheated oven for about 35-50 minutes (depending on the size of the cake tin). Allow cakes to cool.
Chocolate Icing: Melt the butter and chocolate with Guinness in a bain-marie and leave to cool.
Mix the cream cheese, vanilla essence and cocoa together in a large bowl. Mix in a fifth of the icing sugar then add the chocolate mixture. Slowly add more icing sugar until the desired thickness is achieved.
To Assemble: Spread the topping over one of the cake halves then sit the other one on top. Pour the covering over the whole cake.