• Prep (5 minutes):
• Preheat oven to 350°F (176°C)
• Grease and line a 9×5-inch loaf pan with parchment paper
• Position oven rack in center
• Make Batter (10 minutes):
• In medium bowl, whisk together flour, baking soda, baking powder, and salt. Set aside.
• In separate small bowl, mash bananas with fork until mostly smooth.
• In large bowl or stand mixer, cream butter and brown sugar on medium speed for 2-3 minutes until light and fluffy.
• Add eggs one at a time, beating well after each addition.
• Mix in sourdough starter, vanilla extract, sour cream (if using), and mashed bananas until combined.
• With mixer on low, alternately add dry ingredients and milk/oil, beginning and ending with dry ingredients.
• Mix until just combined with a few flour streaks remaining.
• Gently fold in any mix-ins with spatula.
• Bake (55-65 minutes):
• Pour batter into prepared pan and smooth top.
• Optional: Sprinkle extra nuts or chocolate chips on top.
• Bake 55-65 minutes until golden brown and toothpick inserted in center comes out clean or with a few moist crumbs.
• If top browns too quickly, tent loosely with aluminum foil around 40-45 minute mark.
• Cool in pan on wire rack for 15-20 minutes.
• Remove from pan and cool completely on wire rack before slicing (at least 30 minutes).

Notes

• Flour substitutions: Can replace up to 1 cup all-purpose flour with whole wheat, spelt, or einkorn
• Sugar reduction: Can reduce to ¾ cup (150g) without affecting texture significantly
• Dairy-free: Use coconut oil instead of butter and dairy-free milk
• Mini loaves: Bake in three 7×3×2-inch pans for 45 minutes
• Muffins: Makes 18-24 muffins; bake 18-25 minutes
• Storage: Room temperature 3-4 days, refrigerator 7 days, freezer 3 months