Powdered or superfine sugar is simply white granulated or caster sugar that has been ground into a fine powder so that it dissolves easily. It is different to icing sugar as this usually contains cornstarch, wheat flour or calcium phosphate added to prevent icing sugar from forming clumps. If not using sugar syrup, I recommend using powdered sugar instead of granulated or caster sugar.

To make powdered sugar simply grind normal granulated or caster sugar in a mortar and pestle or a coffee grinder.

The use of powdered sugar dates back to the early age of bartending and in his 1862 The Bar-tender's Guide, and Bon-vivant's Companion - A Complete Cyclopedia of Plain And Fancy Drinks , Jerry Thomas specifies a "teaspoon of pulverized white sugar" in cocktails such as the Mint Julep, White Lion and his theatrical Blue Blazer.