Preheat the oven to 180°C.
Line a 12 tin cupcake tray with cupcake cases.
In a large mixing bowl, whisk together all the batter ingredients with an electric hand held whisk until you have a smooth batter; usually 1-2 minutes. Spoon into the prepared cases and bake for 16-20 minutes until springy to the touch and risen. Remove from the oven and allow to cool on wire racks.
Prepare the icing as the cup cakes are cooling; whisk together the butter and icing sugar until combined, smooth and fluffy. Whisk together the cocoa powder and boiling water to make a paste, then add this to the butter and icing sugar mixture and whisk again until combined and even.
Spread on top of the cup cakes once they have cooled, then decorate with the assorted mixed sprinkles before arranging in boxes and serving.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.