You don't need special equipment or exotic ingredients to whip up a batch of your own homemade powdered sugar, which is also commonly called confectioner's sugar or icing sugar. The main thing that differentiates confectioner's sugar from granulated sugar is a finer grind -- and sometimes a little added cornstarch -- which gives it its powdery consistency.
In order to transform the sugar you have on the shelf from crystals to powder, you'll need a high-powered blender or food processor. In a pinch, a spice grinder can be used, although any lingering spice flavors may transfer to the confectioner's sugar. While you're prepping your workspace, gather a measuring cup and measuring spoons.
Processing Granulated Sugar
Combine the sugar and cornstarch in the bowl of your food processor or blender. For optimal grinding power, work in smaller batches. Grind no more than 1/2 cup to 1 cup of granulated sugar at a time. Keep in mind that 1 cup of granulated sugar yields approximately 1 3/4 cups of confectioner's sugar. Add 1 1/2 to 3 teaspoons of cornstarch for each 1 cup of granulated sugar and pulse or blend the mixture for 30 seconds to one minute, or until the sugar has transformed to a fine, fluffy powder.
Because powdered sugar is made of finely ground granules, it's more susceptible to binding with any moisture in the air, which would cause the confectioner's sugar to clump or harden. If you plan to make only enough powdered sugar for immediate use, adding the cornstarch isn't necessary, although it does improve the consistency of the sugar. If you're storing any portion of the confectioner's sugar, add cornstarch to the sugar to prevent caking. Commercially made powdered sugar contains about 3 percent cornstarch to absorb excess moisture.
Storing and Usage
Once you've processed the sugar to the desired consistency, sift the powdered sugar through a sieve or strainer into an airtight container. Storing confectioner's sugar in the refrigerator is not necessary. Instead, keep it in a cool area where the powdered sugar won't be exposed to moisture. In addition to being prone to moisture absorption, the fine granules are also prone to absorbing strong odors, so ensure to keep the container sealed.
Use homemade powdered sugar as you would use its commercially prepared counterpart. For example, sift it over cakes and cookies or combine it with butter to make a smooth, sweet buttercream icing.