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
If you let your sourdough discard sit for several days it will develop a stronger, more tangy flavor. Let it sit in the fridge for 5 to 7 days for the best flavor.
If the starter develops a watery hooch on top then dump the hooch off before adding the starter to the batter. The hooch will add too much liquid to the batter.