In a large bowl, beat the butter and sugar together until soft. Stir in the flour, bringing the mixture together to form a dough. Wrap and chill in the fridge for about 30 minutes.
Preheat the oven to 180ºC/160ºC Fan/Gas 4 and line two baking trays with greaseproof paper.
Once the dough is chilled, roll it out to ½ cm thick and use a 2 cm cookie cutter or a small bottle cap to cut out the biscuit bases. Transfer them to the baking tray.
Bake for 8–10 minutes until lightly golden, then transfer to a wire rack and leave to cool completely.
Make the royal icing by mixing the aquafaba into the icing sugar and stirring until you have a smooth, slightly firm icing. Divide evenly into four bowls and add your chosen food colourings to dye each a different colour, mixing well for an even tint.
Transfer the contents of one bowl to a piping bag with a star-shaped nozzle and pipe a small star onto a quarter of the biscuit bases. Repeat with the next colour until you have four different shades of iced gems.
Leave to set and harden for at least 1 hour before serving. Store in an airtight container for up to 2 weeks.