In a strong food processor or blender add the granulated or Demerara or Turbinado sugar and the cornstarch.
Blend or pulse until the mixture becomes a powder, approximately 3-4 minutes. Rub the mixture between your fingers if it feels like powder with no grainy bits it is done. Sift the powdered sugar and then transfer to an airtight container and store until ready to use.
Notes
Store the powdered sugar in an airtight container in a cool area. It will keep indefinitely. Although if you see mold in the sugar, do not try to remove the mold, throw it all out. If there are bugs or insects, again throw it out. Or if the sugar smells off.I first tried with my food processor but I didn’t find it strong enough so I changed to my strong blender which was perfect. I would recommend using a bullet, a vita mix or what I have is a higher end Moulinex blender, but no where near the price of a vita mix.