Superfine sugar is finely pulverized granulated sugar that dissolves much more quickly than regular granulated sugar. Bartenders use it for its dissolving properties, and it is often called for in light or uncooked desserts such as meringues, mousses, and soufflés. It is not as fine as confectioner’s sugar, which has been crushed mechanically (and generally mixed with a little starch to keep it from clumping). It is called castor (or caster) sugar in Britain.
If your recipe calls for superfine sugar for something that is going to be baked, you can substitute regular sugar. If it is clear that the speedy dissolving is important, you’re better off using superfine.
Domino and other sugar processors package superfine sugar for sale in small boxes, but many grocery stores may not dedicate shelf space for it. Often a store manager will order products he or she doesn't usually stock when a customer requests it.
If you fail in your search and your store manager wouldn’t recognize customer service if he tripped over it, you can make your own superfine sugar by running granulated sugar in a food processor or blender until powdery. After you turn it off, let the powder settle for a minute before you open the food processor. Otherwise you’ll be snorting sugar dust.