Caster or castor sugar is often sold as superfine sugar in the United States. It is a pure white sugar ground down to finer particles than seen in regular table or granulated sugar. It differs from confectioners' or powdered sugar, which typically contains corn starch as well as sugar. Caster sugar got its name because its grains are small enough to slip through the holes of a caster or shaker. Caster sugar commonly appears in recipes for mixed drinks and meringues.
Place granulated sugar in a food processor.
Process for about one minute, until the sugar is a fine powdered consistency.
Wait several minutes before opening the food processor's top to allow the fine powder to settle down.