Preheat oven to 350 degrees Fahrenheit. Line a loaf pan with parchment paper and set aside.
In a large bowl, mash the bananas and measure to ensure it is 1 ½ cups. Add in the wet ingredients: sugar, oil, milk, apple cider vinegar, and vanilla.
Fold in the dry ingredients: flours, flaxseed, salt, baking powder, baking soda, and cinnamon. Mix until just combined; do not overmix.
Scoop out 1 ¾ cups of batter (should be a little less than half) into a separate bowl and add the cocoa powder, 1 tbsp sugar, chocolate chips, and 2 tbsp milk.
Transfer both the chocolate and plain batter to the prepared pan. Place ¼ cup of chocolate batter and ⅓ cup of plain batter in an alternate pattern and repeat until all the batter is used. Lightly swirl with a knife to get the marble effect (do not do too much or they will merge together.
Bake for 45-55 minutes, until a toothpick comes out clean. Let it cool slightly before serving. Enjoy!