Line a small baking dish with parchment paper
Mash the ripe bananas in a mixing bowl until smooth
Add the cocoa powder and mix until fully combined
Adjust the consistency with extra cocoa powder if needed
Add the nut butter and stir until the mixture becomes smooth and glossy
Preheat the oven to 180°C (350°F)
Pour the mixture into the prepared baking dish and spread evenly
Bake for 30 minutes until the surface is set and slightly firm
Let the dessert cool for 10–15 minutes
Lift out using the parchment and cut into squares before serving