Preheat your oven to 350°F (175°C). This ensures that the banana bread bakes evenly.

Grease a 9x5 inch loaf pan with cooking spray or butter to prevent sticking.

In a large mixing bowl, combine the mashed bananas, vegetable oil, eggs, and vanilla extract. Whisk until smooth and well combined.

Add the cake mix and ground cinnamon (if using) to the banana mixture. Stir gently with a rubber spatula until just combined. Be careful not to overmix, as this can make the bread dense.

Fold in the chopped walnuts or pecans if desired, ensuring they are evenly distributed.

Pour the batter into the prepared loaf pan, smoothing the top with the spatula.

Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean. This step is crucial for ensuring the bread is fully cooked.

Once baked, remove the loaf pan from the oven and let it cool in the pan for about 10 minutes.

Carefully remove the banana bread from the pan and transfer it to a wire rack to cool completely before slicing.