Preheat oven to 400°F. Line muffin tin with liners (or grease & flour).
In a large bowl, whisk together flour, baking powder, cinnamon, salt, and nutmeg.
In a medium to large bowl, whisk eggs until mixed. Add in sourdough starter, bananas, oil, sugars, and vanilla. 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 muffin tins. Place into the oven & bake 17-19 minutes, until a toothpick comes out clean. Cool for 15 minutes, then transfer to a rack and cool completely.
Notes
Will keep 3-4 days in an airtight container, or in freezer for 4 months.To make mini muffins, bake for about 12-14 minutes.