Preheat oven 200°C/fan 180°C/Gas Mark 6.
Combine the butter, plain flour, cornflour, caster sugar, and vanilla extract in a food processor and blitz together until just combined.
Add in the egg yolk and blitz briefly to combine.
Remove from the food processor and knead lightly to form a smooth dough. Shape into a thin round and freeze for 5 minutes.
Lightly flour two baking trays. Flour the work surface and then roll the dough out to the thickness of a £1 coin.
Flour a 7cm cutter and stamp out 12 circles, re-rolling the dough if necessary.
Cut out the centre of each biscuit with a 2cm cutter and then transfer the biscuits to the baking trays.
Freeze the biscuits for 5 minutes until firm, and then bake for 10-12 minutes, until baked and very lightly golden.
Once baked, allow to cool for 5 minutes on the tray and then transfer to wire rack.
While the biscuits are baking and cooling, make the icing by whisking together the icing sugar and enough egg whites until you have a smooth icing with a flooding consistency.
Divide into 4 bowls and colour each with the various food colourings. Transfer a third of each colour into 4 separate piping bags.
Working one at a time, place each biscuit onto the surface of the icing in the bowl. You don’t want to dip the whole thing. Lift it back off and use the side of the bowl or spoon to gently scrape off any excess dripping icing. Place it carefully down on the wire rack, icing side facing up.
Choose a piping bag filled with a different coloured royal icing and cut the very tip off. Pipe thin lines over the biscuit, then use a cocktail stick create a feathered effect.
Repeat this process with varying colours and stripes until all the biscuits are iced.
Allow to set and then serve.