Caster sugar is a finer-ground form of the everyday granulated sugar. It is also one of those products that I never seem to have when I need it.
Solution is a pestle and mortar, the required amount of sugar in the regular form, and a few minutes of satisfying thumping.
Although sometimes written as castor sugar, the correct term is caster. A caster is an old-fashioned shaker, found on the tables of the wealthy as far back as the early 1700’s.
Caster sugar dissolves easily, so for finer baking it can be an asset.