Preheat oven to 350°F (175°C). Spray a 9×5-inch loaf pan with nonstick cooking spray.
Mash bananas using a fork or potato masher until mostly lump-free.
In a large mixing bowl, combine mashed bananas, egg, applesauce, and light brown sugar (or sweetener).
In a separate bowl, sift or whisk together all-purpose flour, whole wheat flour (if using), baking soda, salt, and optional cinnamon.
Add the dry ingredients into the wet mixture in batches, mixing just until combined.
If using walnuts or chopped bananas, gently fold them in.
Pour the batter into the prepared loaf pan and bake for 50–60 minutes, or until a toothpick comes out clean.
Allow the loaf to cool in the pan for 10–15 minutes before turning out onto a wire rack.
