A note about substitutions: For dairy-free or vegan versions, replace the whey with vanilla rice or pea protein, and the egg whites with a flax egg. I wouldn't recommend using all whey protein in this recipe. See my
Recipe Basics video for what happens when you use the wrong kind of protein powder in a recipe. Texturally, other nut butters would work here, but they may affect the color of the cookies.
Also, you can get your eggs to room temperature more quickly by placing them in a bowl of warm water.