Preheat your oven to 350°F (175°C) and prepare your loaf pan by greasing it with butter or lining it with parchment paper.

In a medium bowl, whisk together all-purpose flour, baking soda, and salt until well combined. Set aside.

Cream together the softened unsalted butter and brown sugar using a stand mixer until smooth and fluffy, about 2-3 minutes.

Add the whole eggs and sourdough starter to the butter-sugar mixture and mix on medium speed until fully incorporated.

Mash the overripe bananas in a separate bowl until smooth and then gently mix them into the batter.

Gradually fold the dry mixture into the wet batter using a spatula until just combined.

If desired, fold in the chopped walnuts or chocolate chips for additional flavor.

Transfer the batter into the prepared loaf pan, smoothing the top, and bake for 60-65 minutes.

Check for doneness by inserting a toothpick into the center. Let it cool in the pan for 10 minutes before transferring to a wire rack.