Preheat the oven to gas 4, 180°C, fan 160°C.
Put the lemon zest, butter, sugar and flour in a food processor and blend to form fine crumbs. Add the egg yolk and 1 tbsp lemon juice and blend again for a few seconds. Gather into a ball and chill.
Lightly dust a work surface with flour and roll out the dough to the thickness of a £1 coin. Using a round cutter, cut out circles and place on an oiled baking tray. Bake for 10-12 minutes until just golden at the edges. Leave for 10 minutes before removing from the tray to cool.
Make up the royal icing as directed on the pack. Put half in a separate bowl and add a little more water to give a pouring consistency. Colour as desired and keep in airtight containers until needed.
Put the thicker icing in a piping bag with a small plain nozzle and pipe a border around each biscuit. Put the thinner icing in a piping bag and snip the end, then fill in the biscuit with the icing. Leave to set overnight then decorate with more icing and silver balls.