01 - Preheat oven to 350°F. Grease a 9x5-inch loaf pan and line with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt until well blended.
03 - In a large bowl, combine mashed bananas, sugar, oil, peanut butter, eggs, and vanilla. Beat until smooth and creamy.
04 - Add dry ingredients to wet ingredients. Stir gently until just combined; do not overmix.
05 - Fold in chocolate chips until evenly distributed throughout the batter.
06 - Pour batter into prepared loaf pan. Dollop extra peanut butter on top and swirl gently with a knife to create marble pattern.
07 - Bake for 50-55 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.
08 - Cool in pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.