This is the term for sugar with small grains that are between granulated and icing sugar in terms of fineness. It is sometimes spelled castor sugar, and is known as ‘superfine’ sugar.
Caster sugar is obtained from sugar cane or sugar beet, and is valued for its quick-dissolving properties. The usual refined white variety of caster sugar will have been treated to remove molasses, and so will be free-flowing.
Caster sugar is often used in baking cookies, cakes, cupcakes and muffins because of it’s light texture and ability to dissolve easily. It is almost always used when baking and is rarely used raw.
It is NOT used for icing, frostings or buttercream.