People come down on both sides of this debate. There are pastry chefs who swear that cake flour is the only road to tenderness, while others happily use all-purpose (with one crucial addition) for cakes.
It helps to understand why different flours (cake, pastry, bread, and all-purpose) exist in the first place. They all contain different amounts of protein, essential in forming gluten, the thing that gives structure and texture to baked goods. The more protein in a flour, the more gluten the flour forms in a dough or batter. So bread flour, with a protein content ranging from 14 to 16 percent, yields a lot of gluten, which forms the skeletal structure of bread. In contrast, cake flour has a protein content of 7 or 8 percent, which helps makes cakes and other baked goods light and airy. If you’re an occasional home baker, odds are the only type of flour you have in your pantry is all-purpose, which has a protein content of 10 to 12 percent.
This leaves you with a few options. First, you could go out and buy cake flour. But if you don’t think you’ll use it up or can’t otherwise justify the purchase, you could approximate it yourself by removing two tablespoons of all-purpose flour from every cup your recipe calls for and replacing it with 2 tablespoons of cornstarch; sift together before using (author-bloggers Joy Wilson of Joy the Baker and Deb Perelman from Smitten Kitchen both swear by this easy approximation). It works because you’re replacing some of the all-purpose flour that encourages gluten creation with a tenderizing ingredient (cornstarch) that forces the flour to share the liquid in the recipe, moderating gluten development and helping to create a tender cake.
Or you could be like professional baker Warren Brown, owner of Cakelove bakery. He uses all-purpose flour exclusively in cakes, because he feels the extra gluten lends a slightly nutty flavor that cake and pastry flour just can’t achieve. He reduces the all-purpose flour in a cake recipe to the barest minimum needed to maintain structure, then adds bulk with a pure starch (his favorite is potato).
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Leena Trivedi-Grenier is a Bay Area food writer and cooking teacher with an undying love for pot stickers. She earned her master's in gastronomy from Le Cordon Bleu. Besides CHOW, her writing appears on her blog Leena Eats and in various food-based encyclopedias.