- Preheat oven to 350°F; spray a 9-inch square baking dish with nonstick cooking spray.
- Mash bananas with the back of a fork in a large bowl; whisk in eggs and ¾ cup peanut butter. Whisk in flour, sugar, cocoa powder and baking soda; fold in cocoa nibs. Spread banana mixture in prepared dish; drop dollops of remaining ¼ cup peanut butter over banana mixture with a small spoon and swirl peanut butter into banana mixture with a toothpick to create marbled effect.
- Bake banana mixture 30 minutes or until set; cool completely in pan on wire rack.
- Cut brownie into 12 rectangles. Makes 12 brownies.