Preheat oven to 350F. Butter & flour a 9x5 loaf pan, or use baking spray with flour. Set aside.

In a large bowl, mash the bananas. Add in the oil, sugar, milk, vanilla, 1 ½ cups of flour, baking soda, and salt. Mix until combined.

Transfer half the batter to a separate bowl. To one part of the batter, add in the additional ¼ cup of flour and mix to combine. To the other bowl of batter, add in the cocoa powder and chocolate chips. Mix until batter comes together.

Drop spoonfuls of the banana bread batter into the prepared loaf pan, alternating between plain and chocolate. Repeat until all batter has been used up. You can use a butter knife or skewer to swirl the batter around for a more marbled effect. Sprinkle additional chocolate chips on top.

Bake marble banana bread in preheated oven for 40-45 minutes, until a toothpick inserted in the center comes out clean. If the top is browning too fast, you can cover the bread with a piece of foil to prevent burning as it finishes cooking.

Let cool slightly, then remove from pan, slice, and enjoy!