Preheat your oven to 350°F (175°C) and grease a loaf pan.
In a mixing bowl, mash the ripe bananas until smooth.
Add the sourdough starter and melted butter to the bananas and mix well.
Stir in the brown sugar, egg, and vanilla extract until combined.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Pour the batter into the prepared loaf pan, smoothing the top.
Bake for 50-60 minutes or until a toothpick inserted comes out clean.
Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack.
Slice and enjoy your delicious sourdough banana bread!