Castor sugar, or caster sugar, is finely ground granulated table sugar. It sells in the United States at gourmet and specialty food shops and on Amazon.com, either as a baking ingredient or a bartender's essential.
The small granules of castor sugar tend to dissolve quickly and readily in cold fluids, so it is a staple ingredient behind most professional bars. Similarly, the ease and swiftness with which castor sugar dissolves typically yields more favorable results in baking, particularly during the prep stages.
The finer crystals dissolve in room temperature concoctions, which helps to incorporate air into shortening during the creaming process as well as control granularity in a smooth mixture, such as a meringue, a frosting or a sweet soufflé.
Although superfine sugar is available in the states, castor sugar is actually a British equivalent. "Castor" refers to a type of shaker that British food manufacturers package the sugar within. Despite interchangeable references, castor sugar and confectioner's sugar are not the same. Confectioner's sugar is table sugar ground to a powder and mixed with cornstarch.
Processing table sugar for two to three minutes in a food processor fitted with an "S" blade yields the homemade equivalent of castor sugar, according to BeardandBonnet.com. As of 2015, Amazon.com sells 12-ounce bottles and one-pound bags of castor sugar for approximately $8 and $15, respectively.