Please see my Protein Powder FAQ Page (it's in the Notes section of this recipe) for more info. (amyshealthybaking.com)
If you don't have cake flour on hand — see the Notes section of the recipe on how to make your own using all - purpose flour and cornstarch (justsotasty.com)
In the note section of the recipe it says to use rolled oats, which are also known as old fashioned oats! (ambitiouskitchen.com)