There’s something in between granulated and powdered
It’s the end of the year, and that means your weekends for the foreseeable future are likely filled with holiday potlucks, white elephant exchanges, and cookie swaps. That can only mean one thing: you’re probably going to have to bake something. Sure, you can always pick up a box of treats from your local bakery (those sticky, jam-filled, half chocolate-dipped, sprinkled Italian cookies might actually be my favorite food), but say you decide to go homemade. You have a recipe that at first glance looks aligned with your level of baking skill. Then you read more closely and see the recipe calls for caster sugar. Wait, what? Not granulated, not powdered, not brown, but caster sugar.
There is in fact a sugar that is ground finer than granulated sugar (that’s the fine white sugar you probably have somewhere in your kitchen) that is more granular than powdered sugar (the powdery—duh—sugar you’d see dusted over French toast). Caster sugar, also known as superfine sugar, is often called for in recipes where it’s important for sugar granules to dissolve completely, like meringues or syrups, though it’s also often used in cookies and pastries.
Though it’s more common in the UK, caster sugar is available in the United States but might be a bit more expensive than its counterparts. Of course, if you already have a bunch of granulated sugar in your kitchen, you don’t have to go buy a separate bag of superfine sugar. You can make your own.
Pour granulated sugar into a clean coffee grinder or food processor and grind until the sugar is significantly finer, but not so fine that it turns into powdered sugar. Exact timing will depend on the strength of your machine, so it's best to test the texture of the sugar every few seconds. You can then measure out the sugar and follow the recipe as written. Keep in mind that DIY caster sugar will never be as uniquely superfine as store-bought, but this shouldn’t wreck your baked goods.