Step 1: Preheat your oven to 350°F (175°C) and generously grease a 9x5-inch loaf pan with butter or non-stick spray.
Step 2: In a large mixing bowl, combine the melted butter, maple syrup, and brown sugar. Whisk until well combined.
Step 3: Add the egg and vanilla extract to the mixture, whisking until smooth and fully incorporated.
Step 4: Stir in the mashed bananas until evenly distributed throughout the wet ingredients.
Step 5: In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Slowly add the dry ingredients to the wet ingredients, stirring with a spatula just until no streaks of flour remain.
Step 6: Gently fold in the toasted walnuts, ensuring they are evenly spread through the batter.
Step 7: Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Step 8: Bake for 60-65 minutes, or until a toothpick inserted into the center of the loaf comes out clean or with just a few moist crumbs.
Step 9: Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.