Heat the oven to 325 °F and generously grease two loaf pans measuring about 8 1/2 x 4 1/2 inches. Cover the corners and upper sides well so the loaves release cleanly after baking.

Beat the softened butter with the white sugar in a large bowl until the mixture looks lighter in color and fluffy. Properly creamed butter should look smooth rather than grainy and dense.

Mix in the eggs, then stir in the mashed bananas, sour cream, and vanilla extract until evenly blended. The mixture will look creamy and loose with visible soft banana flecks.

Add the flour, salt, cinnamon, and baking soda, then fold gently only until the last flour streaks disappear. The finished batter should be thick and spoonable, not beaten smooth or elastic.

Fold in the walnuts, if using, then divide the batter evenly between the prepared pans. Level the tops lightly so both loaves bake at a similar pace.

Bake for 1 hour, checking the center with a toothpick before removing the pans from the oven. The toothpick should come out clean, and the kitchen should smell of warm banana and caramelized sugar.