Preheat oven to 375°F/190°C. Grease and flour a 9"x 5" loaf pan or line with parchment paper.
Large a bowl, mash bananas with a fork until creamy but not over mashed. You still want to have a few chunks of banana here and there.
Add melted butter, oil, sugar, and brown sugar. Cream together with a spatula until very smooth, about 1 minute. Add eggs one at a time, mixing well between each. Add sour cream and vanilla and mix until smooth.
Add flour, baking soda, baking powder, cinnamon and salt. Mix gently with a spatula. If adding walnuts or other mix ins, stir them while the dry ingredients are still not fully incorporated. Mix only until all traces of dry flour have disappeared. Do not over mix.
Spread batter into prepared loaf pan and place in the oven. Reduce the heat to 350℉/177℃ and bake for 55-60 minutes or until a cake tester inserted in the center comes out clean. Do not over bake.
Cool banana bread in pan for 20 minutes before removing onto cooling rack. Cool completely before serving.