Preheat oven to 350°F. Grease a 9 x 5-inch loaf pan or line it with parchment paper.
Combine the mashed banana, melted butter, sugar, brown sugar, eggs, and sourdough starter in a bowl. Stir well.
Sift the flour, baking powder, baking soda, and salt over the wet mixture.
Gently fold the dry ingredients into the wet mixture.
Pour the batter into the prepared pan.
Split a whole banana in half vertically and place on top of the batter.
Bake 55 to 60 minutes or until a toothpick inserted into the middle comes out clean or with only a few crumbs attached.
Video
Notes
Discard age matters: The longer sourdough discard sits, the more acidic and sour it becomes, which can affect the texture of your banana bread. For best results, use discard that’s no more than 7 days old.If your discard has hooch: That grayish liquid on top? Pour it off before using the discard. Leaving it in adds too much moisture and can throw off the batter.If you like this recipe, try my sourdough banana scones.