Heat the oven to 350°F (175°C). Grease a standard 9 x 5-inch loaf pan and line it with parchment paper if desired, leaving a little overhang to make the baked loaf easier to lift out.

Cream the softened butter and granulated sugar together until the mixture looks light, smooth, and creamy. This usually takes a few minutes with an electric mixer, though a sturdy spoon can also be used.

Beat in the eggs one at a time, mixing after each addition until incorporated. Add the vanilla extract and stir until the mixture is evenly combined.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Distributing the baking soda through the flour helps it work evenly throughout the loaf.

Add the dry ingredients to the butter mixture and stir just until mostly combined. A few flour streaks are fine at this stage because the remaining ingredients will be folded in next.

Fold in the sour cream, mashed very ripe bananas, and chopped walnuts if using. Mix gently and stop as soon as no large pockets of flour remain, since overmixing can make banana bread dense.

Transfer the batter to the prepared loaf pan and smooth the top with a spatula. Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

If the top becomes deeply browned before the center is baked, loosely cover the loaf with foil during the final portion of baking. Let the bread cool in the pan for about 15–20 minutes, then remove it to a rack and cool completely before slicing.