Pre-heat 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.