Preheat your oven to 350°F (175°C) and prepare a greased 9×13-inch baking pan lined with parchment paper.

In a large mixing bowl, mash two overripe bananas until very smooth. Stir in melted butter, sugar, eggs, and vanilla extract until smooth.

In a separate bowl, whisk together flour, baking soda, salt, and cocoa powder then gradually mix into wet ingredients.

Gently stir in milk until batter is thick, then fold in chocolate chips.

Pour batter into the prepared pan, spreading evenly.

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

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

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