I've recently been in Macaron Mania Mode and I've noticed that every recipe calls for pure icing sugar / powdered sugar / confectioners sugar. No big deal, until you discover that the only icing sugar your neighborhood store carries, contains cornstarch or wheat flour in the mix which supposedly improves its flowing ability, therefore very suitable for making icing or frosting ... but not macs. And when you can actually find some confectioners sugar in the fancy bakery supplies store, which is actually pure powdered sugar, it's so not worth paying the price for. Yup, I'm cheap. So, I come up with alternatives.
Just whizz some caster sugar in a dry mill or a food processor until you get the consistency of talc. Sift it if you must.
Easy peasy and no holes in my apron pockets.