Preheat the oven to 350°F (175°C) with the rack in the center position. Keep an eye on the temperature so the loaf bakes evenly.

Grease and line a 9×5-inch loaf pan with parchment paper. Make sure the paper overhang is ready to lift the loaf out cleanly after baking.

Whisk together the all-purpose flour, baking soda, baking powder, salt, and ground cinnamon in a mixing bowl. Whisk until the spices are evenly distributed with no visible clumps.

In a separate bowl, beat the unsalted butter and granulated sugar until light and fluffy. Stop when the mixture looks paler and airy.

Beat in the eggs one at a time until each egg is fully incorporated. The batter should look smooth after the last egg.

Add the vanilla extract, then mix briefly to combine. You should see a uniform color throughout the wet mixture.

Add the mashed ripe bananas and sour cream, then mix until the mixture is cohesive. It may look thick and creamy with banana flecks.

Fold the dry ingredients into the wet ingredients until just combined. Mix only until no dry streaks remain to keep the crumb tender.

Stir in the chopped pecans or walnuts if using. Evenly distribute the nuts without overmixing the batter.

Pour the batter into the prepared loaf pan and sprinkle with coarse sugar if desired. The topping should form a light, sparkly layer on top.

Bake for 55–65 minutes at 350°F (175°C), until a toothpick inserted into the center comes out clean. The loaf should be golden and spring back slightly when touched.

Cool in the pan for 15 minutes before transferring to a wire rack. The loaf should release more easily and be less fragile at this point.

Slice and serve warm. For the classic finish, slice thick and serve with butter.