Preheat oven to 400°F. Prepare a muffin tin by lining with muffin liners.
In a medium bowl, whisk together flour, cocoa powder, salt and baking soda.
In a large bowl, add the flax egg, maple syrup, coconut oil and applesauce. Whisk together until fully combined (about 1 minute). Add the vanilla extract, mashed banana and milk. Whisk together until ingredients become smooth and banana is fully incorporated. Add the dry ingredients to the wet and mix using a spatula or wooden until just incorporated. At the last second, add the apple cider vinegar and mix once more. Do not over mix (this will make your muffins flat).
Fold in the chocolate chips. The batter WILL be thick.
Spoon the batter into the muffin tin. I add about ¼ cup of batter into each cavity. Add a few chocolate chips on top!
Bake muffins for 5 minutes at 400°F. Without opening the oven, decrease the temperature to 350°F and bake for an additional 15-20 minutes (I typically do 17). Remove from oven and let cool for 5 minutes, after 5 minutes place on a cooling rack to cool completely.