Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan by greasing or lining it with parchment paper.

Mash 3-4 ripe bananas in a bowl, then add melted butter, sugar, eggs, and vanilla, whisking until smooth.

In another bowl, whisk together flour, baking soda, salt, and cocoa powder, then fold into the banana mixture until just combined.

Gently add milk and chocolate chips to the batter, ensuring even distribution without overmixing.

Pour the batter into the prepared pan and smooth the surface to ensure even baking.

Combine brown sugar and cinnamon for topping, sprinkle over batter, and dot with pieces of butter.

Bake for 25-30 minutes, checking doneness with a toothpick for moist crumbs.

Cool in the pan for 10 minutes before transferring to a wire rack to cool completely, then slice into squares.