Place the sugar in a small food processor, (clean) coffee grinder or spice grinder.
Grind until it forms a fine powder. Depending on the motor and blade of your appliance, this could take 1-5 minutes. If it does take longer, give it breaks to not burn out the motor. You might need to shake it a little to break up clumps.
Allow it to sit for 1-2 minutes before opening to prevent a powdered sugar bomb from going off.
If using, add the cornstarch by blending in with a few pulses or sifting together.
Store in an airtight container in a cool, dry place for up to 1 year.
If you've tried this recipe, come back and let us know how it was in the comments or ratings.