Preheat oven to 350 degrees F and lightly grease a 9" X 5" loaf pan (or 4 mini pans).
Whisk flour, salt, baking powder, and baking soda together in a bowl and set aside. In a separate bowl, mash the bananas until smooth and set aside.
Melt the butter, and add the brown sugar, eggs, sourdough starter (or discard), vanilla, sour cream, and mashed bananas into a standard mixer with the paddle attachment. Stir until mixture is smooth.
Add the dry ingredients to the wet and mix until just combined. Do not overmix! Gently fold in the nuts or chocolate chips with a spatula if desired.
Pour the batter into the loaf pan and bake for 50-60 minutes if doing one large loaf, or 40 minutes if doing four mini loaves. It is done when a toothpick is inserted into the center and comes out clean.
Allow banana bread to cool in the loaf pan for 10 minutes. Next, gently run a knife along the inside of the pan/s to help release the loaf. Carefully transfer onto a wire rack, and let the loaf rest until it cools down.
Notes
Store at room temperature for up to 3 days, or in freezer for up to 3 months.