Preheat oven to 350°F. Grease or line a 9x5 loaf pan with parchment paper.
In a large bowl, whisk together melted butter, vegetable oil, eggs, granulated sugar, brown sugar, and vanilla until smooth.
Whisk in flour and baking soda until just combined.
Fold in mashed bananas and chocolate chips or walnuts, if using.
Pour batter into prepared pan.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. If the top browns too quickly, loosely cover with foil.
Cool 10–15 minutes before slicing.