01 - Preheat oven to 350°F. Grease a 9x5-inch loaf pan and line with parchment paper.
02 - Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl until evenly 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 mixture. Fold gently until just combined, being careful not to overmix.
05 - Fold in semi-sweet chocolate chips until evenly distributed throughout the batter.
06 - Transfer batter to prepared loaf pan. Drop extra peanut butter in dollops over the surface and swirl gently with a knife to create marble pattern.
07 - Bake for 50 to 55 minutes until a toothpick inserted into the center comes out with just a few moist crumbs.
08 - Let cool in the pan for 10 minutes, then remove to wire rack to cool completely before slicing.