Preheat oven to 325°F and grease a 9x5-inch loaf pan.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together the bananas, brown sugar, sourdough discard, oil, eggs, sour cream, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Pour the batter into the loaf pan. Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean (a few crumbs are okay, but there should be no streaks of wet batter). If the top is browning too much, place a foil tent over the top halfway through.
Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.