Preheat oven to 350°F.
Lightly coat a 9×5-inch loaf pan with olive oil or cooking spray, set aside.
Add bananas, sugar, and vanilla to a bowl. Cream with a hand held mixer or stand mixer fitted with the paddle attachment, about 30 seconds to 1 minute (some small chunks of banana are okay). Alternately, mix by hand until well combined - again, small chunks of banana is ok and welcomed.
Add eggs, one at a time, until fully incorporated. Add the sourdough starter and mix again.
Sift the flour, baking soda, and salt together in a separate bowl. Working in batches, add these dry ingredients to the banana mixture.
Add milk and oil and mix until just combined. Do not over mix; the banana bread will be tough. It's better to leave the batter a bit unmixed than to over mix.
Pour the batter into the prepared pan.
Bake for 60- 65 minutes (for the standard 9×5-inch pan) or 25-35 for muffins, until rich golden brown and a toothpick comes out clean. Cover with foil if the loaf browns too quickly.
Cool in the pan for 20 minutes, and then transfer to a wire rack to cool completely.
Serve and enjoy!