Powdered sugar has cornstarch in it. If you are like me and can't have corn, you can make your own powdered sugar for frostings in a coffee grinder. I use a Cuisinart mini-chopper/grinder with the sharp blade side down. This is a great inexpensive little kitchen gadget that is useful for mincing fresh herbs and onions, or powdering dried herbs, or grinding rice for cream of rice, etc.

Put sugar in grinder in small amounts, about 1/4 cup at a time. Grind until it becomes powdered. You will end up with more than what you started with.

The cornstarch used in commercial powdered sugar is to keep it flowing or to keep it from lumping. How it works is that the powdered sugar absorbs moisture from the air and clumps. Your homemade powdered sugar will not be as fine as commercial powdered sugar, so you probably won't need something to substitute for the cornstarch unless you are worried about it clumping if you store it. In that case, I've read you can add 1 Tbsp of arrowroot or potato starch to 1 cup of homemade powdered sugar.

Claire