Start with all-purpose flour
Measure 1 cup (120 g) all-purpose flour
Remove 2 tablespoons (16 g) from the measured flour
Add 2 tablespoons (16 g) cornstarch
Sift the mixture 5–7 times until evenly blended
Use immediately or store in an airtight container for up to 1 month
For 1 cup cake flour (120 g): use the substitution above (1 cup all-purpose flour minus 2 tbsp + 2 tbsp cornstarch)