1. Preheat the oven to 350F.
2. In a large bowl, combine banana, sugar, oil, milk and vanilla, whisking until smooth. Add flour, baking soda, salt, chocolate chips.
3. Mix gently. Careful not to overmix the batter!
4. Scoop out 1 cup of batter and transfer it to a small bowl. In another small bowl, mix the cocoa powder with 3 tablespoons of boiling water & stir until chocolate dissolves.
5. Add this chocolate mixture to the 1 cup batter and mix until the chocolate is smooth and incorporated.
6. Back to the plain banana batter in the large bowl. Add 3 tablespoons of boiling water and mix batter until relatively smooth.
7. Lightly grease a 4x8 inch loaf pan. Scoop alternate 1/2 cupfuls of chocolate batter and plain batter into the loaf pan. The more random the better. Take a butter knife and swirl it through the batter in circular directions for a few seconds.
8. Bake for 55 minutes. If the toothpick comes out batter-free, it’s done.
9. Let cool on a cooling rack.