You can easily substitute cake flour with a mixture of all-purpose flour and cornstarch. Start with one level cup of AP flour, remove two tablespoons of the flour, and add two tablespoons of cornstarch. Sift the mixture together to be sure the ingredients are well distributed. In summary: 1 cup AP flour – 2 Tablespoons AP flour + 2 Tablespoons cornstarch = 1 cup cake flour