Preheat oven to 350°F (175°C), grease and line loaf pan.
Mash bananas in a bowl until smooth.
Stir in melted butter, brown sugar, egg, vanilla, and sour cream.
In another bowl, whisk flour, baking soda, salt, and cinnamon.
Gently fold dry ingredients into wet mixture.
Add chocolate chips and walnuts, reserving some chips for topping.
Pour batter into loaf pan, sprinkle remaining chips.
Bake 55–65 min, toothpick should come out with moist crumbs.
Cool 10 min in pan, then transfer to wire rack.