Place the sugar into the mixing bowl and mill for 20 sec/speed 10. Wait for a few seconds for the icing sugar to settle. Scrape down the sides of the bowl with a spatula. Mill again for 30 sec/speed 10.
Transfer the icing sugar to an airtight container and store in the pantry.
Notes
Ensure the Thermomix mixing bowl and blades are completely dry before milling.
Milling up to 250g of sugar at a time ensures the best results.