Heat the oven to 180C / Gas 4. Grease a 12 hole mini mu‑n tray with butter.
Put the butter and sugar into a bowl and beat with a handheld electric whisk until soft and fluffy. Beat in the egg until evenly combined.
Add the flour, cocoa powder, bicarbonate of soda and baking powder and beat briefly until well incorporated. Now add the blackcurrant cordial and chocolate chips and mix well. Finally stir in the Guinness.
Divide the mixture between the mini muffin moulds and bake for 12-15 minutes, until the mini muffins are well risen and a skewer inserted in the centre comes out clean. Leave to cool in the tin for 5 minutes, then run a knife around the edge of each one and turn out onto a wire rack to cool completely.
For the topping, beat the mascarpone and icing sugar together in a bowl with a wooden spoon until smooth. Spread on top of the muffins with a small palette knife.