As a professional baker, I always have powdered sugar on hand. In my line of work, it's safe to assume that at a moment's notice, I'll need to dust a torte, whip meringue, toss puppy chow together, or make chocolate fudge. It's not that I regularly make my own powdered sugar (I promise I'm not THAT person) — or that buying some is particularly strenuous or expensive — but it's handy to know how to make homemade powdered sugar for two big reasons.
Sometimes you find yourself without confectioners' sugar and the thought of running to the grocery store doesn't appeal in the slightest. Maybe it's a snow day and you need some to make homemade marshmallows for a cup of hot cocoa, or it's hours away from your little one's birthday and you're frantically trying to whip buttercream frosting for a birthday cake. Maybe you thought you had enough and then the little bag of sugar had half a cup shy of what you were expecting. Or you want the results of powdered sugar with something other than granulated white sugar (I've made it with raw cane sugar and coconut sugar, but you can turn nearly any type of granulated sugar into powdered in minutes). This may be because you'd rather use natural sweeteners or to satisfy a dietary concern or that's what you have to use up. Either way, the flavor won't be affected. You can use brown sugar for a richer molasses flavor, but know that you won't achieve a pure white fluffy frosting; instead, it will have a light brown undertone.
So what exactly is powdered sugar (or confectioners' sugar or icing sugar) anyway? Whatever you want to call it, powdered sugar is just white sugar that has been finely ground into a powder. When it's made commercially, it has starch added to prevent caking, but it also plays a role in recipes: Think about royal icing or a basic glaze. They start out fluid and soft, but set up shiny and firm (that's all thanks to the starch!). To make your own powdered sugar at home, that's all you've got to do — grind up some granulated sugar in a powerful blender like a Vitamix or food processor and add a small amount of starch.
Here's how to do it:
You can use any one of the following tools: a clean/dry coffee or spice grinder, a high-power blender, or a food processor. The coffee/spice grinder is the most efficient, but obviously can only handle a small amount of sugar at a time, so a high powered blender or a food processor is the next best thing.
Weigh out an amount of sugar, and add it to the blender or food processor (1/2 cup of granulated sugar will make about 1 cup powdered sugar, but that will vary slightly based on the type of sugar you use). Pulse on high speed until the sugar is no longer visibly granulated, but instead resembles a fine powder. This can take 2 to 5 minutes, depending on the amount of sugar and the machine you're using. Give yourself (and the machine) a break so that neither of you get burnt out.
Transfer your now-powdered sugar to a large sieve and add 3% of the sugar's original weight in cornstarch. So if you started with 10 ounces of sugar, add 0.30 ounces of cornstarch, and sift the two together 2 to 3 times to combine. Cornstarch is naturally gluten-free, but you can also use arrowroot starch/powder, which will create a super fine, super smooth sugar. You can store the powdered sugar in an airtight container, or use it immediately as you would the store-bought kind. If stored properly, homemade powdered sugar should keep for one to two years. Frosting, anyone?
The good news is that if you want to make whipped cream, you don't need powdered sugar. You can use regular granulated sugar or vanilla extract for sweetness. But now you'll never be caught short of powdered sugar again.