Even if you’re no stranger to baking a morning batch of yeast donuts or desserts for the end of a meal, caster sugar is most likely not a common ingredient in your baking cabinet.
If you haven’t heard the name or even what it’s called in the US, i.e., “superfine sugar,” that’s because it inhabits an odd middle ground between granulated and powdered sugar.
In fact, although caster sugar is a useful ingredient, most American bakers are only familiar with it because of its occasional appearance on “The Great British Bake Off.”