Always measure your flour by weight if you have a scale. One cup of all-purpose flour weighs about 125 grams. Remove 16 grams of flour, then add 16 grams of cornstarch.
If you’re using the spoon-and-level method, do not pack the flour into your measuring cup. Fluff it first with a fork, then spoon it in gently—this prevents the densest, heaviest cakes of your life.
Another little-known trick: if you only have self-rising flour, skip the cornstarch. Simply use the same flour but omit any baking powder and salt the recipe calls for. It’s a sneaky one-step workaround.