In a bowl, cream together the butter, caster sugar and cinnamon until light and fluffy. Add the egg and flour and mix to a dough. Gather into a ball, wrap in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 200°C, fan 180°C, gas 6. On a lightly floured surface, roll out the dough to a thickness of 1⁄2 cm, then cut out star and Christmas tree shapes with cutters. Transfer to two non-stick baking trays and bake for 12-14 minutes. Cool on a wire rack.
Mix the icing sugar with 1-2 tablespoons of water in a bowl until smooth and thick. Spoon into a piping bag fitted with a writing icing nozzle. Pipe decorations on to the biscuits then decorate with the sprinkles, silver balls and pearls, and dust with icing sugar.