Preheat oven to 160 degrees celsius (fan-forced). Line two large flat baking trays with baking paper and set aside.
Place the icing sugar into the Thermomix bowl and mix on Speed 8, 10 seconds. Scrape down the sides of the bowl and add the butter and plain flour.
Mix on Speed 6, 10-15 seconds and then on Interval/Knead function for 1 minute.
Gently knead the dough into a ball (but don't overwork it) and then roll out to 1/2-1 cm thick. Use cookie cutters to cut the dough shapes out.
Place the shapes onto the prepared trays and bake for 12-15 minutes or until only just very lightly golden (do not overcook).
Allow to cool on the trays for 10 minutes before transferring to a wire rack to cool completely.