Add the sugar to the Thermomix bowl and mill for 20 seconds on speed 10. Let the fine sugar settle briefly before opening the lid. Scrape down the sides with a spatula, then mill again for 30 seconds on speed 10.
After milling, the icing sugar may be slightly warm. Let it cool thoroughly before storing it in an airtight container to avoid clumping. Sieve the icing sugar before using for best results.
Notes
Icing Sugar Tips: Use regular white sugar (I use Homebrand). Avoid brown sugar, as it's sticky and tends to clump. Raw or coconut sugar can work but may be coarser. Let the sugar settle before opening the lid to avoid a dust cloud. For best results, blitz no more than 250g at a time.