Preheat the oven to 325℉. Grease a 9x5-inch loaf pan with non-stick cooking spray or line with parchment paper.

In a large mixing bowl, whisk together the oil, brown sugar, eggs, vanilla, and mashed bananas. Fold in the flour, baking soda, and salt just until incorporated, then fold in the chocolate chips. Do not overmix.

Transfer about 1 ½ cups of batter to another bowl and fold in the cocoa powder. Fill the prepared pan, alternating with a scoop of regular batter and then a scoop of chocolate batter, and so on to create a marbled effect. Swirl a butter knife through the filled pan to further swirl and marble the batter, being careful not to overmix.

Bake in the preheated oven for 60-75 minutes or until a toothpick inserted into the center of the loaf comes out clean. Let cool in the pan for 10 minutes before transferring to a wire rack to finish cooling.