Preheat your oven to 350°F (175°C). Grease an 8x8 inch baking pan and line it with parchment paper, leaving an overhang on two sides to easily lift the brownies out later.

In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined.

Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract.

In a separate small bowl, whisk together the all-purpose flour, cocoa powder, and salt. Add the dry ingredients to the wet ingredients and fold with a spatula until just combined. Do not overmix. Fold in the chocolate chips.

In a separate medium bowl, whisk together the mashed bananas, light brown sugar, egg, and melted butter.

In another small bowl, whisk the flour, baking soda, cinnamon, and salt. Add this to the banana mixture and stir until just combined.

Pour about two-thirds of the brownie batter into the prepared pan and spread it into an even layer.

Dollop the entire banana bread batter over the brownie layer. Then, dollop the remaining brownie batter in the gaps between the banana bread batter.

Using a butter knife or a skewer, gently swirl the batters together to create a marbled effect. Be careful not to overmix, as you want distinct layers of flavor.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with moist, fudgy crumbs attached (not wet batter).

Let the brownies cool completely in the pan on a wire rack before using the parchment paper overhang to lift them out. For the cleanest cuts, chill them in the refrigerator for 30 minutes before slicing.