Mix filling ingredients, and refrigerate to set while you bake the cookies.
Mix all cookie ingredients until well combined.
Spoon cookie batter onto a parchment-lined cookie sheet (as 8 cookies) and use your spoon to shape them into circles.
Bake for 8-10 minutes, just until the cookies are firm. Be sure not to overbake!
Let cookies cool.
Turn over four cookies and pipe or spoon filling on top.
Cover with the other cookies, and enjoy!
Store extras in the refrigerator.
Notes
A note about substitutions: I wouldn't recommend substituting other types of protein powder in this recipe (whey concentrate would be best). The macros per 1 cookie (of 8) without any filling: Prot: 8g, Carbs: 5g (1g fiber, 2g sugar), Fat: 3g, Cal: 80