In a food processor, add the butter and sugar and process until combined.
Add the egg and vanilla and process until combined.
In a separate bowl, sift the flour, baking powder and salt and stir to combine.
Add the flour mix to the food processor gradually until it is all mixed and a dough ball forms.
Shape this into a disk and refrigerate for 1 hour.
Preheat the oven to 160°C. Cover a baking sheet with baking paper and set aside.
Roll out the disk on a flour dusted suface using a rolling pin and roll it to a 5mm thickness. Dip your cutter into some flour and cut your shapes out, placing them on the tray. Sprinkle with caster sugar.
Bake for 25-30 minutes until slightly golden. Leave to cool for 5 minutes on the tray and cool and then transfer to a wire rack to finish cooling.
Using a hand mixer, mix the egg whites until frothy, add the lemon juice and gradually add the sifted icing sugar until fully combined.
Pour icing mixture into four bowls and colour each one with a touch of food colouring. Using a knife, spread half the icing over the biscuits. Pour the remaining icing into ziplock bags to use as piping bags and snip the tip off and decorate the iced biscuits. Add sugared decorations.