01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, combine mashed bananas, sugar, melted butter, egg, and vanilla extract. Mix until smooth.
03 - Add flour, baking powder, baking soda, and salt to the banana mixture. Stir just until combined. Do not overmix.
04 - In a separate bowl, whisk together melted butter and sugar. Add eggs and vanilla, mixing until glossy.
05 - Sift in cocoa powder, flour, and salt. Stir until just combined. Fold in chocolate chips.
06 - Spread half of the banana batter evenly in the prepared pan. Spoon the brownie batter over it, then dollop remaining banana batter on top.
07 - Use a knife or skewer to swirl the batters together, creating a marbled effect.
08 - Bake for 28-32 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
09 - Cool completely in the pan before lifting out and slicing into 16 bars.