Preheat the oven to 350°F (175°C). Grease and lightly flour a 9×5-inch loaf pan.

In a large bowl, beat the softened butter and sugar until the mixture is pale and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the mashed bananas, sour cream, and vanilla until evenly combined.

In a separate bowl, whisk together the flour, baking soda, and salt.

Add the dry ingredients to the wet mixture in two additions, folding gently until just combined. Do not overmix.

Fold in the chopped nuts if using.

Pour the batter into the prepared loaf pan and smooth the top.

Bake for 60–70 minutes, starting to check at 55 minutes with a toothpick inserted in the center. It should come out clean or with a few moist crumbs.

Cool the loaf in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.