Spray a 12 cup muffin tin with cooking spray, or fill with muffin liners. Preheat your oven to 425°. Mash bananas with a fork.
In a large bowl, whisk together the melted butter, oil, eggs, sour cream, mashed bananas, and vanilla.
Whisk the flour, sugar, baking soda, baking powder, and salt in a separate bowl. Stir gently into the banana mixture just until there are no streaks of flour.
Use an ice cream scoop to divide the muffin batter between the muffin cups.
Bake muffins for 5 minutes at 425°, then turn the oven temperature down to 350°. Bake for another 10-12 minutes, or until a toothpick inserted in the middle comes out clean and the edges are golden brown.
Let them cool in the muffin pan for about 5-8 minutes, then transfer them to a wire rack. We like them best served warm!