The optional part of making powdered sugar is the adding of the starch, either arrowroot or cornstarch. The cornstarch is added by manufacturers to prevent clumping. You don't have to add the cornstarch or arrowroot if you are going to use it right away.
The recipe I use for powdered sugar is below. Put all the ingredients in your blender. Turn on high, until it is powdered sugar. It takes a little less than a minute.
- 1 cup of granulated sugar
- 1 teaspoon of arrowroot or cornstarch (optional)