Mug Cakes by Mima Sinclair © 2014 Kyle Books, and the photographs © Tara Fisher. No images may be used, in print or electronically, without written consent from the publisher.
Place the butter and chocolate in a 1 ½ cup mug and microwave until melted, 10 to 20 seconds. Add the egg to the mug and beat with a fork until combined. Stir though the Guinness, then add the sugar, flour, baking powder, and salt and beat again until smooth. Cook in the microwave for 2 minutes at 600W, 1 minute 45 seconds at 800W or 1 minute 30 seconds at 1000W. Leave to cool.
Beat the sugar and cream cheese together until smooth, then spread over the cooled cake, swirling with the back of a spoon. Scatter with the sprinkles to serve.