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

Mash the ripe bananas in a large mixing bowl until smooth. Mix in melted butter, then stir in granulated sugar, eggs, and vanilla extract.

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

Gently fold in the milk and chocolate chips until evenly incorporated. The batter should be thick and fudgy.

Pour the brownie batter into the prepared pan and spread it evenly using a spatula.

Mix brown sugar and ground cinnamon in a small bowl. Sprinkle over the brownie batter, then dot with small pieces of butter.

Bake for 25-30 minutes until a toothpick comes out with a few moist crumbs.

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