Preheat the oven to 180 C / Gas 4. Line a baking tray with baking parchment.
Whisk the egg yolks with the caster sugar until pale. Whisk in the butter. Stir in the plain flour and baking powder to make a soft dough.
Roll the dough between 2 sheets of baking parchment, and cut with biscuit cutters in any shape you would like. Place the biscuits on the prepared baking tray.
Bake in the preheated oven until the edges of the biscuits begin to turn golden, about 15 minutes. Remove to cool on a wire rack for 10 minutes.
Meanwhile, stir the icing sugar together with a drop of water to form a paste. Stir in the food colouring. With a paintbrush, draw any design you'd like: witches, bats, mummies, skeletons - everything is fair game for Halloween!