Preheat your oven to 350°F (175°C). Grease the 9x9-inch baking pan or line it with parchment paper for easier removal.
In a mixing bowl, combine the mashed bananas and melted butter. Mix until well combined.
Add sugar, eggs, and vanilla extract to the banana mixture. Whisk thoroughly until smooth.
In another bowl, whisk together the flour, cocoa powder, baking soda, salt, and cinnamon if using.
Gradually fold the dry ingredients into the wet mixture using a rubber spatula or wooden spoon until just combined. Do not overmix.
If desired, fold in the chocolate chips for added richness.
Pour the batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for about 25 minutes or until a toothpick inserted in the center comes out mostly clean (a few moist crumbs are okay).
Let the brownies cool in the pan for about 10-15 minutes before slicing into 9 squares. Enjoy your Banana Bread Brownies!