Pre-heat oven to 350 degrees F. Line a small 8x8 baking dish or 8'' skillet with parchment paper or grease well. Set aside.
In a large bowl, combine the mashed bananas, peanut butter, whisk until fully combined. Stir in protein powder and cocoa powder with a wooden spoon. Pour batter into a greased pan. Bake for 12-20 minutes or until cooked through.
Cool for at least 10 minutes before cutting. Cover the leftovers and store in the fridge for up to 3 days.