Prepare Oven and Pan: Adjust oven rack to lower-middle position. Preheat oven to 350°F (175°C). Generously coat a 9x5-inch loaf pan with vegetable oil spray.
Combine Dry Ingredients: In a large bowl, whisk together flour, sugar, baking soda, and salt. Set aside.
Combine Wet Ingredients: In a separate medium bowl, whisk together mashed bananas, melted butter, eggs, yogurt, and vanilla extract until smooth.
Gently Combine Mixtures: Pour wet banana mixture into dry flour mixture. Gently fold with a rubber spatula until just combined. Do NOT overmix; batter will be thick and chunky.
Fold in Walnuts (Optional): If using, gently fold in toasted and chopped walnuts.
Transfer to Pan: Scrape batter into prepared pan and smooth the top.
Bake: Bake for 55-65 minutes, or until golden brown and a toothpick inserted into the center comes out with just a few crumbs attached.
Cooling: Let loaf cool in the pan on a wire rack for 10 minutes.
Unmold and Finish Cooling: Invert loaf onto wire rack and cool completely for at least 1 hour before slicing.