Preheat Oven & Prepare Pan
Preheat oven to 350°F (175°C). Grease and lightly flour a 9x5-inch loaf pan or line it with parchment paper.
Mix Dry Ingredients
In a medium bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon. Set aside.
Cream Butter & Sugar
In a large bowl, beat butter and sugar together until light and fluffy.
Add Eggs & Flavor
Beat in eggs one at a time, then mix in vanilla extract.
Add Bananas & Sour Cream
Stir in mashed bananas and sour cream until well combined.
Combine Wet & Dry
Gradually add dry ingredients to the wet mixture, mixing just until combined.
Fold in Nuts
Gently fold in the chopped walnuts or pecans.
Bake
Pour batter into prepared loaf pan. Sprinkle optional topping nuts over batter.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean.
Cool & Serve
Let bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.