Preheat oven to 350 degrees.

Lightly grease a 9×5 loaf pan with butter.

Combine dry ingredients (flour, baking soda, baking powder, and salt) well in a medium bowl.

In a stand mixer or bowl, mix melted butter and brown sugar for five minutes until light and fluffy.

Add in eggs, vanilla, mashed bananas, and sourdough starter.

Fold in dry ingredients a bit at a time until just incorporated. Be careful not to over-mix.

Gently fold in any other add-ins you would like (pecans, walnuts, and chocolate chips or chunks, etc.).

Pour into a greased pan. You could also line your loaf pan with parchment paper.

Bake for 50-60 minutes. Test bread at 50 minutes with a toothpick or cake tester.

Allow to cool in the pan for one hour.

12-24 hours before you plan to make banana bread, mix together sourdough starter, melted butter, eggs, and flour. Cover with a towel or plastic wrap and allow to ferment at room temperature for 6 hours. Put the batter in the fridge and allow it to ferment for another 6-18 hours.

The next day, preheat oven to 350 degrees.

Using a stand mixer with the paddle attachment, add brown sugar, vanilla, mashed bananas, salt, baking powder and baking soda to mixing bowl.

I highly recommend using a stand mixer because the fermented dough will be stiff and difficult to combine with the other ingredients by hand.

Set on low to medium speed until completely combined.

Pour into loaf pan and bake for 50-60 minutes.

Allow to cool in the pan for one hour.