Preheat oven to 350F (175C). Line the bottom of an 8-inch (20 cm) square baking pan with parchment paper, letting the paper hang over two sides. Set aside.In a food processor, place bananas and process until smooth (you also can make this without a food processor by mashing banana with a potato masher). When smooth add cocoa powder, peanut butter and blend until smooth. At this stage you also can add sweetener.Pour the batter into prepared pan and bake for about 30 minutes.Allow to cool slightly and cut into squares.