Spoon mixture into cases and bake at 180°C, 350°F, Gas Mark 4 for 20-25 minutes until spongy to the touch and golden brown. Leave to cool.
Meanwhile mix your icing sugar with a little water until smooth a quite thick.
To serve spread a little icing onto the cooled fairy cakes and finish with a topping of your choice, e.g. Chocolate sprinkles.