Preheat the oven to 350°F (180°C), or 340°F (170°C) for a fan oven. Line a 9 x 13-inch (23 x 33 cm) baking pan with parchment paper, leaving enough overhang to lift the brownies out later. You can also lightly grease the pan.

Place the butter and sugar in a large microwave-safe bowl. Microwave for about 45 seconds at medium power, then stir well. Continue heating in short intervals and stirring until the butter is completely melted. Alternatively, gently melt the mixture using a double boiler.

Set the butter mixture aside for a few minutes so it can cool slightly. Whisk in the eggs and vanilla until smooth and combined.

Add the mashed ripe bananas and stir until evenly incorporated.

Sift the cocoa powder if it has any lumps. Add the flour, cocoa powder, and salt to the wet mixture. Gently whisk or stir just until combined. Avoid overmixing. Small lumps from the mashed bananas are completely normal.

Fold the chocolate chips evenly into the batter.

Spread the batter into the prepared baking pan, smoothing the top into an even layer.

Bake the Banana Brownies for approximately 35 minutes. Check for doneness by inserting a toothpick into the center. It should come out with a few moist crumbs rather than wet batter.

Let the brownies cool completely in the pan before slicing. For the neatest squares and the best fudgy texture, allow them several hours to cool and set.