Preheat your oven to 350°F (175°C) and grease or line a 9x13-inch baking pan with parchment paper.

In a large mixing bowl, mash the ripe bananas until smooth, mixing in melted butter, granulated sugar, eggs, and vanilla until creamy.

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

Gently fold in milk and chocolate chips if using, taking care not to overmix.

Pour the brownie batter into the prepared pan, spreading it evenly.

Mix brown sugar and cinnamon in a bowl and sprinkle over the batter, dotting with small pieces of butter.

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

Let cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.