Cake: Preheat oven to 180 degrees C. Line two 22cm cake tins with baking paper and spray with cooking spray.
Pour the beer into a saucepan then add the butter and bring to a simmer over medium-low heat. Remove the saucepan from the heat and whisk in the cocoa powder until the mixture is smooth. Allow the mixture to cool.
In a bowl whisk the eggs and sour cream together until smooth with an electric mixer and stir in the stout mixture to make a smooth thick liquid.
In a large bowl mix together the flour, sugar, bicarbonate of soda and salt. Pour the stout mixture into the flour mixture and gently combine with a spatula. Pour the batter into the prepared cake tins.
Bake in the preheated oven until the cakes are set and a toothpick inserted into the centre of a cake comes out clean; about 30 minutes. Remove and let cool in tins for 5 minutes before inverting the cakes onto wire racks to finish cooling.
Icing: Mix together the icing sugar, butter and Irish cream liqueur until the mixture forms a smooth and spreadable icing (add more sugar or liqueur as needed to create the desired consistency). Spread half the icing on top of each cake.
Chocolate Ganache: Place the chopped chocolate into a heatproof bowl. Bring cream to almost boiling in a small saucepan and pour over the chocolate. Add the butter then stir the mixture until the chocolate melts and the mixture is very smooth. Allow to cool until the mixture is pourable but not thin or overly runny; 10 to 15 minutes.
To Assemble Stack a cake gently on top of the other, icing sides uppermost and carefully drizzle the chocolate mixture over the cake allowing decorative drips to run down the sides.