Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan.
In a large bowl, mash the bananas.
In a separate bowl, whisk together flour, baking soda, and salt.
In another bowl, mix egg, sugar, melted butter, milk, and vanilla extract.
Combine wet ingredients with mashed bananas.
Gradually fold dry ingredients into the banana mixture.
Gently stir in chopped walnuts if using.
Pour batter into the prepared loaf pan.
Bake for 60-65 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.