Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large bowl, whisk together the all-purpose flour, baking soda, and salt until well combined.
In a separate medium bowl, mix the mashed bananas, granulated sugar, beaten egg, melted butter, and vanilla extract until smooth.
Pour the banana mixture into the flour mixture and stir just until combined. Do not overmix; the batter should be slightly lumpy.
Fold in the chopped walnuts gently into the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.