Preheat oven to 350 degrees Fahrenheit. Grease 5" x 9" loaf pan or line with parchment paper and set aside.
In a small bowl, whisk together the flax seed and water to make your flax egg. Let sit 2-3 minutes to form a gel.
Add the bananas to a large mixing bowl bowl and mash well with a fork.
Add the flax egg, oil, maple syrup, milk, and vanilla extract to the bowl with the mashed bananas and stir well.
In a small mixing bowl, add the dry ingredients (flour, baking soda, baking powder, salt) and whisk to mix them together.
Stir dry ingredients into wet ingredients just until mixed, it's ok to have some lumps still.
In a small bowl, add the chocolate chips and microwave for 1 minute. Stir and heat another 30 seconds or so until melted. Measure out 1 cup of the bread dough and stir into the melted chocolate.
Pour 1/2 of the banana bread mixture into pan. Use spoon to add 1/2 of the chocolate dough. Create a swirl pattern with butter knife. Repeat with the rest of the dough, swirling again.
Top with a few extra chocolate chips if desired.
Bake for 45-55 minutes or until tooth pick interested into center of loaf comes out clean. Allow to cool completely before slicing.