To make the biscuits preheat the oven to 180C/160C fan/gas 4 and prepare two lined baking trays. Beat the butter and sugar in a bowl until well combined. Now beat in the egg and vanilla. Add all of the flour and combine into a soft dough. If the dough is very soft, chill for 15 mins.
Roll the dough to about 3mm thick and cut around the Ubercorn template with a sharp knife. Keep cutting out Ubercorn shapes, re-rolling the offcuts until you have 30 biscuits. Cut holes in the centre of 10 of the biscuits using a 4cm round cutter. Transfer to the lined trays and bake for 8-10 mins until golden brown, then transfer onto a wire rack to cool.
Follow the packet instructions to make up the royal icing and add extra water one tablespoon at a time until smooth enough to pipe with. We used about 80ml water. Keep half the icing white, dye 1/4 dark purple for the mane, and divide the remaining 1/4 between four bowls. Dye them yellow for the horn; lighter purple for the mask outline; very light lilac for the ears and nose; and bright pink for mask detail.
Put half of the white icing into a separate bowl and gradually add water to make it a slighly runnier consistency and put this in a piping bag. Then do the same with half of the dark purple icing. These will be used as fillings for the face and mane.
For the outline/detail icings make up six piping bags with number two nozzles (or snip a small piece off the end of each piping bag). Put one colour in each bag: white, dark purple, light purple, very light lilac, bright pink, yellow.
Now ice 10 of the biscuits. First off, outline Ubercorn's face and ears with the white outline icing, then fill in with the runnier white. Leave this to set for at least two hours, or overnight if you prefer. Next, pipe the outline for the mane and flood in with the runnier dark purple. Now add the yellow horn detail and continue by piping over the mask and small face details. For the eyebrows and eyes, use a tiny piece of black sugar paste to mould these with your fingers.
To create the piñata effect, sandwich the cookies together and fill with sprinkles. To do this, pipe around the back of an un-iced ubercorn biscuit with a little leftover royal icing. Place a biscuit with the hole cut out on top, press down and fill the hole with sprinkles. Then pipe another line of royal icing around the edge and sandwich the decorated biscuit on top. Leave to dry for a few hours before wrapping or serving.