Prep. Preheat the oven to 350°F. Butter a 9x5-inch loaf pan, or line with parchment.

Mix the wet ingredients. In a large bowl, whisk the mashed bananas (340 g), melted butter (113 g / 8 Tbsp), brown sugar (150 g / ¾ cup), and granulated sugar (50 g / ¼ cup) until smooth.

Add the eggs and sour cream. Whisk in the eggs, sour cream (120 g / ½ cup), and vanilla (10 g / 2 tsp) until fully combined.

Add the dry ingredients. Sprinkle the flour (300 g / 2½ cups), baking soda (7 g / 1½ tsp), salt (5 g / ¾ tsp), and cinnamon (3 g / ½ tsp) over the batter and fold just until no dry streaks remain — a few lumps are fine.

Fold in the nuts. If using, fold in the walnuts or pecans (100 g / 1 cup).

Bake. Pour the batter into the prepared pan and bake 55–65 minutes, until a toothpick inserted in the center comes out with a few moist crumbs, not wet batter.

Cool. Let the loaf cool in the pan 15 minutes, then turn out onto a rack and cool at least 1 hour before slicing.