Preheat the oven to 360 F.
Grease a 9x5 inch loaf pan with coconut oil.
Add the bananas, brown sugar, and vanilla to the bowl of a stand mixer fitted with the paddle attachment and mix until combined.
Add the eggs and the sourdough starter and blend until combined.
In a separate bowl, mix together the flour, baking soda, baking powder and salt.
Slowly add the dry mixture to the wet mixture, mixing between additions until all the dry ingredients are incorporated.
Add the milk and oil and mix until just combined - but no longer.
Add the chocolate chips and fold into the batter.
Pour the batter into the greased pan.
Bake for about 60 minutes until a toothpick comes out clean. After about 40 minutes of cooking check the top of the loaf to make sure it's not getting too brown. If so, place a piece of foil overtop to prevent burning.
Cool in the pan for 15 minutes, then transfer to a cooling rack.