Preheat and Prepare:
Preheat your oven to 350°F (175°C).
Grease and line a 9x5-inch loaf pan with parchment paper.
Mix the Wet Ingredients:
In a large bowl, whisk together the mashed bananas, oil, brown sugar, granulated sugar, sourdough starter, eggs, and vanilla extract until smooth.
Combine Dry Ingredients:
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
Fold and Combine:
Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Avoid overmixing to keep the bread tender.
Pour and Bake:
Pour the batter into the prepared loaf pan. Smooth the top and place it in the oven.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Enjoy:
Let the bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.