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 completely before storing (see notes below).
Notes
Icing sugar tips: Use regular white sugar (any supermarket brand works perfectly). Avoid brown sugar, as it's sticky and tends to clump. Raw or coconut sugar can work but may be coarser. For best results, blitz no more than 250g at a time. After milling the sugar, let it settle for a few seconds before slowly opening the lid - this prevents a dust cloud.Storage: The icing sugar may feel slightly warm after milling. Let it cool completely, then store in an airtight container to prevent clumping. Sift before using for best results.