Preheat oven to 325 degrees. Line your 9x9 baking dish with parchment paper and set it aside. You don't have to do this, it just makes it easier to take it from the pan after baking. If you don't have it, or don't want to use it, spray your pan with pan spray or brush with butter.
Combine your flour, salt, protein powder, and cocoa powder into a small bowl, mix well, and set aside.
Place your eggs in a medium-sized bowl and whip them on high for about 4 minutes.
Then add in the sugar and vanilla and mix for another minute. Add your peanut butter, butter, and chocolate to a small bowl. Microwave for about 30-45 seconds then mix well.
Add the peanut butter mix to the eggs and mix until they are combined. Last, add in the flour mixture and mix onto to combine.
Pour it into the prepared pan and place it in the oven. Bake for 40-45 minutes.
Remove and allow it to cool for about 5 minutes in the pan, then carefully lift the parchment out and set it on a cooling rack.
These are delicious warm, and just as good cooled! Enjoy!