– 3 large very ripe bananas, mashed – 1/2 teaspoon ground cinnamon – 1/4 cup melted butter, cooled down a bit – 1/4 cup vegetable oil – 2 large eggs, close to room temperature – 1 teaspoon vanilla extract – 1 (15 ounce) box yellow cake mix (do not add the other ingredients listed on the box) – 1 cup chopped walnuts, pecans or mini chocolate chips (optional)