Mix flour, icing sugar, salt, lemon and lime zest and vanilla in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the egg, mix quickly to create a soft biscuit dough (do not knead), cover and refrigerate for approx. 1 hr.
On a lightly floured surface, roll out the dough in batches to approx. 7 mm thick. Cut out the biscuits (e.g. rabbits, eggs). Place the biscuits on two baking trays lined with baking paper. Chill again for approx. 15 mins.
Approx. 13 mins. per tray in the centre of an oven preheated to 200°C. Remove from the oven, leave to cool on a cooling rack.
Mix the icing sugar with the lemon juice. Dip the surface of the biscuits in the icing, leave to dry for approx. 30 mins. Colour the remaining icing, transfer the coloured icing to separate disposable piping bags, cut off the tip of each bag, decorate the biscuits with dots and lines, leave to dry.