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.