Preheat oven to 375°F. Grease and flour two 8½ x 4½ inch loaf pans.
In a large bowl, whisk together flour, baking powder, cinnamon, and salt.
In a medium to large bowl, whisk eggs until mixed. Add in sourdough starter, bananas, oil, sugars, and vanilla and whisk until well combined.
Add wet mixture to the flour mixture and fold, scraping bottom and sides using a rubber spatula, until just mixed. Do not overmix or banana bread will be tough.
Divide batter evenly between two prepared loaf pans. Place pans into the oven and reduce temperature to 350˚F. Bake 35-45 minutes, until a toothpick comes out clean. Cool for 15 minutes, then transfer to a rack and cool completely.
Will keep 3-4 days in an airtight container or freeze for 4 months.