Preheat your oven to 350°F (175°C) and grease a loaf pan.
In a large bowl, mash the ripe bananas until smooth.
Add the sourdough starter and mix well.
Stir in the melted butter until fully incorporated.
Add brown sugar, egg, and vanilla extract; mix until combined.
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in walnuts or chocolate chips if desired.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 60 minutes, or until a toothpick comes out clean.