Preheat oven to 350°F (175°C). Grease and line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides.

To make the brownie batter, whisk together melted butter and granulated sugar in a large bowl. Beat in the eggs one at a time, then stir in the vanilla. In a separate bowl, whisk flour, cocoa powder, and salt. Add the dry ingredients to the wet ingredients and mix until just combined.

For the banana bread batter, mash the ripe banana in a medium bowl. Whisk in the brown sugar, egg, and melted butter. In a small bowl, combine the flour and baking soda, then add this to the banana mixture, stirring until just incorporated.

Pour about 2/3 of the brownie batter into the prepared pan and spread evenly. Drop spoonfuls of the banana bread batter on top. Add the remaining brownie batter in dollops.

Use a knife to gently swirl the batters together to create a marbled effect. Be careful not to overmix.

Bake for 28-32 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached, but not wet batter.

Let the brownies cool completely in the pan before lifting them out with the parchment paper and slicing into 16 squares.