Preheat oven 180°C/350°F and butter and line 2 x 23cm (9") springform tins.
Melt the butter into the Guinness in a saucepan over low heat.
Whisk in the cocoa and sugar and take the saucepan off the heat.
Beat the sour cream with the eggs and vanilla in a separate bowl until combined, than add to the beer mixture.
Whisk in the flour and bicarb until combined.
Divide the cake batter between the two tins and bake for 35-45 minutes. The middle of the cake may still be slightly wet when you take the cake out, but it will firm up as it cools down.
Leave to cool completely in the tin as it is quite a damp cake and could collapse.
Since these cakes need to be sliced horizontally, I wrap both cakes in cling wrap and store in the fridge for a few hours to firm up. It makes slicing the cakes easier.