Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Peel the bananas and add them to a food processor together with the peanut butter and cocoa powder.
Blitz until you get a smooth consistency.
Grease a 6-hole muffin tray/small square baking tray with oil or butter, and distribute the batter evenly.
Top with chocolate chips and chopped nuts - optional.
Bake for 20-25 minutes until the edges and the top are firm and the brownies are well risen.
Remove from the oven and leave to cool down completely.
