Preheat your oven to 350°F (175°C). Grease or line a small baking dish with parchment paper.
In a medium bowl, mash the bananas until smooth.
Add cocoa powder and nut butter, stirring until the mixture is thick and well combined.
Spread the batter evenly into the prepared baking dish.
Bake for about 20-25 minutes, or until the top is set and edges look firm.
Allow to cool before slicing into squares and enjoying.