Add the white sugar to the Thermomix bowl and mix for 2 sec/speed 8.
Store in a cool, dry place.
Notes
The result won't look identical to store-bought caster sugar - it may differ in texture and appear whiter. Commercial caster sugar is milled to a precise grain size using industrial equipment, which a Thermomix can't replicate exactly. It still works just as well in baking.
For best results, process 200g of white sugar at a time.
White sugar is white sugar - supermarket home brand works just as well and costs significantly less.
In the UK, regular white sugar is typically labelled as granulated sugar.