Preheat oven 350 degrees F.
Coat 9x5 pan with cooking spray; set aside.
In a large mixing bowl cream butter.
Add sugars and cream 3-4 minutes until light and fluffy.
Add egg; mix 2 minutes.
Add vanilla, sourdough start, and mashed banana; mix to incorporate.
In a separate bowl combine flour, sea salt, and baking soda; use a whisk to combine.
Add flour mixture all at once to wet ingredients; mix until 80% incorporated.
Add mini semi-sweet chocolate chips; gently mix with spatula to combine.
Do not over mix- a few lumps are okay.
Evenly spread batter into prepared pan.
Bake 45-60 minutes until a toothpick inserted in the center removes clean.
If at 45 minutes the top of the loaf looks very browned, set a piece of foil over the top to protect over browning. ( I needed to do this)
Cool on rack for 10 minutes, turn out and complete cooling.