1. Preheat oven to 350°F (175°C). Grease a 9×5-inch loaf pan generously with butter and dust with flour, or line with parchment paper.

2. Whisk together flour, baking soda, salt, and cinnamon in a medium bowl. Set aside.

3. In a large bowl, mash bananas with a fork until mostly smooth with a few small chunks (about 1½ cups).

4. Add sugar to mashed bananas and whisk vigorously for 30 seconds to combine.

5. Add eggs one at a time, whisking well after each addition until light and frothy.

6. Whisk in melted butter, vanilla extract, and sour cream until completely smooth.

7. Add dry ingredients all at once and fold gently with rubber spatula just until no flour streaks remain (15-20 folds).

8. If using mix-ins, fold in with 3-4 gentle strokes.

9. Pour batter into prepared pan and spread evenly. Tap pan gently on counter twice.

10. Bake for 55-65 minutes until toothpick inserted in center comes out with just a few moist crumbs.

11. Cool in pan for 15 minutes, then run knife around edges and invert onto cooling rack.

12. Cool completely before slicing, at least 30 minutes.