Preheat the oven to 180°C, fan 160°C, gas mark 4.
Rub together the unsalted butter and plain flour in a mixing bowl. Add the caster sugar and bring the mixture together with your hands to form a stiff dough.
Roll out the dough to 0.5cm thickness and cut out circle shapes with a 6cm cutter. You may need to re-roll the trimmings to use up all the dough.
Place on a baking tray and bake in the preheated oven for 10-12 minutes until golden. Leave to cool on the baking tray for a few mins, then carefully transfer to wire racks to cool completely.
Roll out the icing to 0.5cm thickness, then use the same 6cm cutter to cut out icing rounds.
Gently warm about 2 tablespoons of the apricot jam in the microwave. Spread a little of the warm jam over each biscuit to act as glue for the icing, then top with the icing rounds.
Use the writing pens to decorate as desired for a fun and tasty treat.