Preheat oven to 350°F (180°C or 170°C fan-forced).

Line a 9x13 inch (23x33 cm) baking pan with parchment paper or foil, leaving an overhang for easy lifting. Lightly grease if preferred.

In a large heatproof bowl, combine butter and sugar. Microwave for 45 seconds on medium power, then stir. Repeat until the butter is fully melted and the mixture is glossy. Alternatively, melt the butter and sugar over a double boiler.

Let the mixture cool slightly. Whisk in the eggs and vanilla until smooth. Then stir in the mashed bananas until well combined.

Sift the cocoa powder if needed to remove lumps. Whisk together the flour, cocoa, and salt, then gently fold into the wet mixture. The batter will be thick with banana lumps—that's okay.

Fold in the chocolate chips, reserving a few to sprinkle on top if desired.

Spread the batter evenly in the prepared pan. Bake for about 35 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter).

Cool the brownies in the pan. Then refrigerate for at least 4 hours before slicing to get the best texture and clean cuts.