Preheat oven to 180 C / Gas 4. Grease a 23cm springform cake tin.
Pour beer into a large saucepan over medium-low heat. Add 255g sliced butter; heat until butter is melted, about 5 minutes. Whisk in caster sugar and cocoa powder until dissolved. Remove from heat.
Mix soured cream, eggs and 1 tablespoon vanilla extract together in a bowl. Stir into beer mixture. Whisk flour and bicarbonate of soda into the beer mixture until mixture is smooth; pour into the greased cake tin.
Bake in the preheated oven until a skewer inserted into the centre comes out clean, 45 to 60 minutes. Run a knife around the edge of the cake; cool to room temperature on a wire rack, about 30 minutes. Remove ring and transfer cake to a serving platter.
Beat 115g butter and cream cheese together in a bowl. Add vanilla extract. Mix in icing sugar slowly until icing is smooth. Add liqueur 1 tablespoon at a time to thin the icing to a spreading consistency. Spread icing over cooled cake.