Preheat the oven to 180C/350F and line a square brownie tin with parchment paper
Mash bananas in a mixing bowl, then add peanut butter and cocoa powder
Mix until a batter forms, adding a splash of milk if too dry
Fold in chocolate chips if desired
Transfer batter to the lined pan and bake for 25 to 30 minutes
Allow to cool in the pan before slicing