Spoon 1 cup (120 grams) of all-purpose flour into a dry measuring cup and level it off with a knife.

Measure 2 tablespoons (16 grams) of cornstarch using a measuring spoon.

Combine the measured flour and cornstarch in a small mixing bowl or glass jar.

Use a fork or small whisk to stir vigorously for about 30 seconds until the cornstarch is fully incorporated and no white streaks remain.

Sift the mixture through a fine-mesh strainer or sifter into another bowl, repeating 2 to 3 times to aerate and remove lumps.

Transfer the finished cake flour to an airtight container and label with the date.

Store in a cool, dark pantry for up to three months or freeze in a freezer-safe container for up to one year.

Use this homemade cake flour as a one-to-one substitute for commercial cake flour in your favorite cake recipes.