Put 1 Tbsp potato starch in a cup and fill the rest with cane sugar. Put that in a blender and pulse it until you have a fine powder. Don't run it continuously or you will burn out your blender and the sugar will cake onto the blade. Shake the container around every now and then. This can take quite a while, so patience is key here. The consistency is usually still a bit grainy.
Notes
This makes corn-free confectioner's sugar that can be used in frosting recipes.
This can also be done in a coffee grinder but needs to be done in smaller batches. You can get a finer grind in a coffee mill.
Substitutions
This would probably work with any safe starch such as tapioca or arrowrrot.
Corn Substitutions: Corn is a common ingredient in products. Starch, modified food starch, dextrin and maltodextrin can be from corn. Consult with your physician to find out which corn derivatives you need to avoid. Many corn-free options are available in the US. Find out more about corn substitutions.