1. Preheat oven to 350°F (175°C). Grease a 9×5-inch loaf pan with butter or spray, then line with parchment for easy release. Cream butter and sugar: In a large bowl, beat softened butter and sugar with a hand mixer or wooden spoon for 2-3 minutes until light and fluffy. This traps air for a tender crumb. Add eggs and vanilla: Beat in eggs one at a time, then vanilla, scraping the bowl after each. Mash and mix bananas: Peel overripe bananas and mash with a fork until smooth but chunky. Stir into the batter until combined. Incorporate sour cream: Gently fold in sour cream for extra moisture. Combine dry ingredients: In a separate bowl, whisk flour, baking soda, baking powder, and salt. Add to wet mixture in three additions, stirring gently just until no streaks remain. Fold in nuts or chips if using. Avoid overmixing to prevent toughness. Bake: Pour batter into pan, smooth top. Bake 55-65 minutes, tenting with foil at 40 minutes if browning too fast. A toothpick inserted in center should emerge clean or with moist crumbs. Cool properly: Let rest in pan 10 minutes, then transfer to a wire rack. Cool completely before slicing, about 2 hours.