This cake us utterly fabulous, light, moist, not overly sweet and absolutely moreish with the Baileys cream.

Ingredients
250g butter cut into chunks 
250ml Guinness 
80g cocoa powder 
275 g plain flour 
350g soft light brown sugar 
1 tsp bicarbonate of soda 
150 ml buttermilk 
3 eggs 
1 tsp vanilla extract or vanilla bean paste 
100g chocolate chips (optional) 

Icing Ingredients 
250g mascarpone COLD 
250 ml whipping cream COLD 
100g icing sugar 
3 tbsps Baileys, or to taste 

Method
Preheat the oven to 160c and line a deep 23cm round tin. 

In a saucepan, heat the butter and Guinness until the butter has melted. 

In a large bowl or jug stir together all of the dry ingredients, the cocoa powder, flour, sugar and bicarb. 

Add the butter mixture and stir until fully incorporated, then the buttermilk and stir again until fully incorporated and then the eggs and vanilla and mix again.  If you are using chocolate chips add these now and stir through.  Put the mixture into the prepared tin and put into the oven for 45 minutes before checking with a cocktail stick.  The stick should come out clean.  If it does not, put it back in for another 5 minutes or so before checking again. 

Once baked, leave to cool fully in the tins. 

When the cake is cooked, put the mascarpone into a bowl and with a wooden spoon, loosen the mascarpone - don't go too far or it will turn liquid so as soon as it softens add the icing sugar and stir.  Meanwhile put the cream and Baileys into a stand mixer or with an electric whisk beat until the cream is in soft peaks.  Do not overmix as you need to stir through the mascarpone and you don't want it to split.  Fold the mascarpone through the cream until fully incorporated. 

Put the icing over the top of the cake to mimic the top of a pint of Guinness.