Preheat oven to 350° Fahrenheit and line a 9x4-inch loaf pan with parchment paper, then grease the inside of the pan with oil or butter.
Mash the banana in a large bowl until mostly smooth, then add in the peanut butter, chocolate protein powder, and cocoa powder. Mix everything together until fully combined.
Transfer the brownie batter to your prepared baking pan and top it with chocolate chips (if using), then bake for 18-20 minutes or until the brownies are just set in the middle.
Remove the pan from the oven and let the brownies cool completely before slicing.