Preheat oven to 350℉.
In a small bowl, combine the ground flaxseed with 6 Tbsp. of water, mix well, and set aside to let thicken slightly (5-7 minutes).
Cream together the butter and sugar until light and fluffy, about 5 minutes.
Add the flax and vanilla and mix an additional minute.
Sift together the dry ingredients (flour and baking soda). Add half of the flour to the butter mixture, mix, then add the milk, mix. Add other half of flour and mix until just combined (the mixture will be very thick but be sure not to overmix it!)
Fold in the mashed bananas. Divide this batter into two separate bowls. To one bowl, add the chocolate chips. To the other bowl, add the cocoa powder (make sure to sift it first so it doesn't get clumpy).
In a parchment-lined 9"x5" loaf pan, alternate adding the two batters. My favorite way to get the marbled effect is to add scoops of each batter randomly in the loaf pan, then use a knife to gently swirl the layers together.
Bake 65-70 minutes until fully baked. Because of the chocolate, it might be hard to tell, but the regular banana bread sections should be golden brown and a toothpick inserted in the middle of the loaf will come out clean.