For the short pastry, knead butter in pieces, 1 pinch of salt, sugar, egg yolk, lemon zest and 240 g flour to a smooth dough. Wrap in foil and chill for about 1 hour.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line two baking trays with baking paper. Knead dough on a little flour and roll out to a thickness of approx. 5 mm. Cut out the cake biscuits using a template.
Knead the rest of the dough again, roll out and cut out until the dough is used up, then place on the trays. Bake in the oven for 12-15 minutes one after the other. Let it cool down.
For the decoration, mix icing sugar and egg white with the whisks of the mixer to a glaze. Ca. 1⁄3 Pour the icing into a piping bag and set aside. Stir the rest of the icing with lemon juice to make it more fluid.
Cut this cast in half, one half green, the other half blue. Fill one piping bag each, cut off small tips and paint the biscuits with them, leaving the edge free all around.
Allow to dry. Cut a small tip off the piping bag with the rest of the white icing and squirt around the edges of the cake biscuits. Leave to dry. Decorate the biscuits with the remaining white icing as desired.