What is cake flour?
Cake flour is milled very finely and contains less gluten-forming protein than bread or all-purpose flour. It supports the soft, even crumb wanted in layer cakes, sponge cakes, and other tender bakes rather than the chew expected from bread flour.
The fine powder is easy to compact and should be kept sealed against humidity. Break up lumps, spoon it lightly, and level once; the adopted result is about 120 grams per US cup.