Preheat the oven to 350°F (175°C).
Line a small baking dish with parchment paper.
In a medium bowl, mash the bananas until smooth.
Stir in the cocoa powder and nut butter until well combined.
Spread the mixture evenly into the prepared dish.
Bake for 20 to 25 minutes, until set but still fudgy in the middle.
Let cool before slicing into squares.