Preheat your oven to 350 degrees Fahrenheit and lightly grease an 8×4-inch loaf pan with butter or cooking spray.
In a large bowl, mash together the ripe bananas until they are relatively smooth. In a medium bowl, mix together the eggs, butter, vanilla extract, and sugar until well blended. Next add in the mashed bananas and sour cream, whisking until everything is evenly distributed.
In a separate bowl, combine the flour, baking soda, and salt; stir in the chopped walnuts or pecans and mix until evenly distributed.
Slowly add the dry ingredients to the wet ingredients and mix just until combined. Take care to not overmix as this can lead to a rubbery texture.
Pour the batter into the greased loaf pan and spread it evenly. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool for 10 minutes before removing from the pan.