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

Mash the overripe bananas in a large bowl until smooth, then mix in the melted butter, sugar, eggs, and vanilla until creamy.

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

Gently fold in the milk and chocolate chips, being careful not to overmix.

Spread the brownie batter evenly into the prepared pan and top with a mixture of brown sugar and cinnamon, along with small pieces of butter.

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

Let the brownies cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.