Prep: Line a 9×5-inch pan with a parchment sling and lightly grease. Preheat oven to 350°F (175°C), rack in center.
Mash: In a large bowl mash bananas until mostly smooth.
Wet mix: Whisk in brown sugar, eggs, and vanilla. Whisk in melted butter (or oil) and sour cream/yogurt until glossy.
Dry mix: In a second bowl whisk flour, baking soda, and salt.
Combine: Add dry to wet in two additions. Fold gently just until the flour disappears; a few small lumps are fine.
Chips: Fold in chocolate chips (and nuts, if using). Reserve a spoonful for the top.
Pan: Transfer batter to the pan and level the top; sprinkle reserved chips.
Bake: 55–65 minutes, starting to check at 50. Tent with foil if the top browns early.
Doneness: Toothpick near center should show a few moist crumbs, or internal temp 200–205°F (93–96°C).
Cool & slice: Cool in pan 15 minutes. Lift to a rack and cool until just warm. Slice with a serrated knife.