Pre-heat oven on Gas mark 4. Put paper cases into a 12 hole muffin tin.

Beat the butter and sugar together until combined then add the eggs and flour until fully incorporated.

Melt the chocolate in a ban-marie over a pan then add to the mixture along with the cocoa powder and mix until incorporated. Then add the Guinness and mix again until combined

Using an ice cream scoop, scoop the mixture into the cases. Then bake in the oven until golden brown or when a skewer comes out clean.

Mix the butter until pale and fluffy then gradually add the icing sugar until incorporated.

Add the Guinness a tablespoon at a time until you have the right consistency.