Cake flour is soft wheat flour with low protein content. What does this mean to a novice baker? It’s more forgiving than all-purpose flour. If you accidentally overmix the batter, the cake’s texture won’t be tough after baking. Less gluten is developed, and for cakes this is a critical factor―it means a soft, tender crumb. Cake flour also is bleached, which explains its very bright, white appearance.
If you use all-purpose flour instead of cake flour, reduce the amount of flour called for by 2 Tbsp. per cup. Or make your own version of cake flour by whisking together 3/4 cup all-purpose flour plus 2 Tbsp cornstarch for each cup of cake flour.