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

Grease a 9x5 inch loaf pan with cooking spray or butter. This prevents the bread from sticking to the pan.

In a large mixing bowl, mash the ripe bananas with a fork until smooth. This creates the base for your banana bread and adds natural sweetness.

Add the yellow cake mix, vegetable oil, eggs, vanilla extract, baking soda, and salt to the mashed bananas. Stir with a whisk until just combined. Be careful not to overmix, as this can make the bread tough.

Fold in the chopped walnuts if using. This adds a nice crunch and flavor to the bread.

Pour the batter into the prepared loaf pan, smoothing the top with a spatula. This helps it bake evenly.

Place the loaf pan in the preheated oven and bake for 55-60 minutes, or until a toothpick inserted into the center comes out clean. This indicates that the bread is fully cooked.

Once baked, remove the bread from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. This prevents the bottom from becoming soggy.

Slice the banana bread once it has cooled completely. Serve it plain or with butter for a delicious treat!