Making a cake flour substitute is easy with the following two ingredients: all-purpose flour and either cornstarch or arrowroot powder.
1 cup AP flour – 2 Tablespoons AP flour + 2 Tablespoons cornstarch or arrowroot = 1 cup cake flour
Start with one level cup of AP flour, remove two tablespoons of the flour, and add two tablespoons of cornstarch or arrowroot powder back in. Then sift the mixture together to be sure the ingredients are well distributed.
When added to all-purpose flour, cornstarch will inhibit the formation of gluten while also giving structure and “sponginess” to your cake. While cornstarch can easily be swapped for arrowroot powder, it is important to note that arrowroot will cause cakes to cook more quickly, and will often be moister than made with cornstarch.