- Preheat your oven to 350°F. 
- Grease two 8×4-inch loaf pans and set aside. 
- In a large bowl, combine all-purpose flour, sugar, brown sugar, baking powder, ground cinnamon, baking soda, and salt. 
- Mix everything thoroughly. 
- In another bowl, mix the softened butter, vegetable oil, vanilla extract, eggs, and mashed bananas. 
- Stir until smooth and well blended. 
- Gently fold the wet ingredients into the dry ingredients. 
- Be careful not to overmix; fold just until combined. 
- Pour batter into the greased loaf pans, filling each halfway. 
- Sprinkle walnuts on top, if using, and bake for 45 to 55 minutes or until a toothpick inserted comes out clean.