Place a sifter over a large bowl, and add the 2 cups gluten free flour and 4 tablespoons cornstarch. Sift into the bowl until blended.
Notes
Does this method work with nut flour? I haven't tried this with almond flour, but I think it would work fine. Keep in mind that nut flours are higher-protein flours, so even with the addition of cornstarch, the cake will not be quite as light as a cake made with oat or rice flour.
Increase the quantities. This recipe makes 2 cups of gluten free cake flour. Feel free to double or triple the quantities if you'd like to keep a larger batch on hand.
Can I use a gluten-free flour blend? Yes, you can use a 1:1 gluten free all-purpose flour instead of a single-origin flour. Most blends already contain a starchy flour, so adding cornstarch may make your bakes quite delicate.
Measure by weight. For the best results, measure the gluten-free flour out by weight so you have complete accuracy. Otherwise, use the spoon and level method to measure the flour.
Per recipe: If you want to quickly do a DIY cake flour substitute per recipe, measure out the flour needed. Then, figure out how many tablespoons to remove and replace (example: 1 cup of flour = remove 2 tablespoons and replace with 2 tablespoons of cornstarch; 1 ½ cups of flour = remove 3 tablespoons and replace with 3 tablespoons of cornstarch; 2 cups of flour = remove 4 tablespoons of flour and replace with 4 tablespoons of cornstarch, etc).