Cake flour contains less protein than all-purpose flour, which results in a more tender crumb in a baked cake because less gluten is formed. If you don't have cake flour available, you can make a substitute with all-purpose flour and cornstarch. The cornstarch inhibits gluten from forming, so it works well for moist or heavier cakes. Cake flour substitutions don't work as well for light cakes with exceptionally tender crumb, such as angel food.
Measure the all-purpose flour into a mixing bowl. Use the same amount of all-purpose flour as the amount of cake flour indicated in the recipe.
Remove 2 tablespoons for every cup of flour in the bowl. For example, if the recipe requires 2 ½ cups of cake flour, remove 5 tablespoons of all-purpose flour.
Replace the removed all-purpose flour with 2 tablespoons of cornstarch for every 1 cup of flour. Cornstarch prevents gluten from forming, resulting in a more tender cake similar to the results achieved when baking with cake flour.
Whisk the all-purpose and cake flour together, combining them completely and evenly. Alternatively, sift the flour and cornstarch together in a flour sifter to mix them together. Mix and bake the cake according to the recipe.