Add the granulated sugar to a dry, high-speed blender or food processor.
Blend for 30 seconds to 1 minute, or until the sugar becomes very fine and fluffy.
Optional: Add 1 tablespoon of cornstarch per cup of granulated sugar, and pulse to combine, especially if you're storing it for later.
Let the dust settle before opening the lid, then check the texture.
If needed, sift through a fine mesh strainer for extra smoothness.
Use the powdered sugar immediately or store it in an airtight container.