In a large bowl, combine protein powder, peanut butter or powdered peanut butter, cocoa powder, erythritol, and vanilla extract and stir until completely combined. If using regular peanut butter (and not powdered), use your fingers to knead the dough, ensuring that the dry ingredients are incorporated into the peanut butter as well as possible.
Now here comes the part where you really have to pay attention. Because protein powders vary so much in texture and ability to hold liquid, add in the almond milk one tablespoon at a time, mixing thoroughly between each addition and stopping when you've reached a cookie dough texture. Obviously you will end up using slightly more if you use powdered peanut butter instead of regular peanut butter, as peanut butter is wet but the powder is dry.
Using a heaping ½ tablespoon measure, roll the dough into balls and flatten into disks about 1-1/2 inches long. I flatten these on a baking sheet lined with parchment paper, but do whatever is easiest for you.
For cookie dough and assembly:
In a large bowl, combine peanut butter, protein powder, and erythritol and stir until completely combined. Add in enough almond milk so that the dry ingredients are totally incorporated. See note in brownie batter description about variation in protein powders and how much liquid to add. Stir in the coconut flour. Again, SLOWLY add in almond milk until you have a perfect cookie dough texture. Stir in chopped dark chocolate.
Using a 1 teaspoon measure, roll the cookie dough into balls. Place a ball in the center of each brownie batter disk, fold the brownie batter around the ball, and press gently to seal. If not serving immediately, stick the baking sheet/plate/whatever surface you’re using into the fridge for a few hours before transferring them to a zip lock bag for storage. They last for a few days in the fridge. Devour.