To make superfine sugar at home, put the amount of granulated white sugar called for in your recipe in a food processor. Add a couple of tablespoons more to compensate for a reduction in overall volume. Process for 1 to 2 minutes until the sugar feels like fine sand, letting the sugar dust settle before opening your food processor to check. (If you use a high-powered countertop appliance like a Vitamix or Blendtec, it goes much faster, usually within a couple pulses.) Remeasure before using.