Preheat the oven to 170°C (150°C fan, gas mark 3)
Line cupcake tine with 12 cases.
Pour Guinness into a saucepan.
Add butter, heat gently and stir until the butter has melted.
Remove from the heat and add the cocoa powder and sugar.
In a separate bowl add vanilla extract and buttermilk.
Add this mixture to saucepan.
In a large bowl, sift together flour, bicarbonate of soda and baking powder.
Add saucepan contents to large bowl.
Mix well using an electric whisk. i
Spoon the mixture into the cupcake cases
Bake for approx 25 mins.
Remove cakes from the oven.
Leave the cakes on a cooling wire to cool completely.
Mix together the butter and icing sugar .
Add cream cheese.
Spoon onto top of cakes.