In a small bowl, mix together apple cider vinegar and milk, and set aside for 10 minutes to curdle.
Line the bottom of a 9 x 5 inch loaf pan with parchment paper and set aside. Slightly overhang the paper on the sides - it helps to remove the bread easily from the pan.
Preheat the oven to 350℉/180℃
In a mixing bowl, mix together the dry ingredients - all purpose flour, cocoa powder, baking soda, cinnamon powder, and salt.
In a large mixing bowl, mash the bananas with a fork.
Add the brown sugar to the mashed bananas and mix them together.
Combine some oil with the mashed bananas and mix everything well.
Transfer the curdled milk, gently mix with the banana mixture, and add some vanilla extract.
Combine the dry ingredients into 2 batches to the wet ingredients and gently mix. Do not over mix the batter.
Add half of the chocolate chips to the bread batter, mix, and transfer it to a lined baking tray.
Evenly spread the batter with a spatula and sprinkle the remaining chocolate chips all over the top of the bread.
Gently tap the pan 1-2 times on a countertop and bake for 50-60 minutes, or until the bread comes out clean when a toothpick is inserted in the middle.
Transfer the baking tray onto a cooling rack for about 15 minutes.
Gently lift the parchment paper from a baking pan and let the bread cool completely on a cooling rack.
Transfer the bread to a cutting board and evenly cut into 12 pieces.