Place the ingredients in a blender, food processor or Thermomix and blend on high speed for 30-60 seconds or until it forms a fine powder.
Use straight away or store in an air-tight container.
Notes
Corn flour can be excluded. It's there to keep the icing sugar from clumping if you store it. You can always just give it a quick blend if it does clump.
This method can be use with white or raw sugar or coconut or rapadura sugar (they last two won't turn perfect white).