Preheat the oven to 350°F (175°C) and grease a 9×5-inch loaf pan or line with parchment paper.
In a large mixing bowl, mash the bananas and set aside.
In another bowl, cream together the butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each, then stir in the vanilla extract.
Mix in the mashed bananas and sour cream until fully combined.
In a separate bowl, whisk together flour, baking soda, and salt.
Gradually add the dry ingredients to the banana mixture, stirring just until combined.
Fold in chopped walnuts or chocolate chips if using.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.