Preheat oven to 350°F. Prepare a 9x5-inch loaf pan with non-stick spray.
In a medium bowl combine the flour, baking soda, and salt. Stir with a fork to combine ingredients. Set aside.
In a large bowl (or bowl of your stand mixer) add the butter and sugars. Mix on medium-high with a hand-held mixer until ingredients are fully incorporated, about 1-2 minutes.
To the bowl add in the eggs, vanilla, and bananas. Mix very well until only a few lumps of banana remain. May need to scrape the bowl.
Add in the flour mixture and the sourdough starter and stir gently by hand.
Pour batter into prepared pan.
Bake at 350°F for 55-65 minutes or until an inserted toothpick is removed with crumbs but no wet batter. Do not overbake.
Allow to cool for at least 10 minutes before cutting.