Preheat the oven to 350°F.

In a large mixing bowl, mash the bananas and mix in the melted butter using a wooden spoon. Stir in the sugar, egg, and vanilla until well combined.

Sprinkle the baking soda and salt over the mixture and gently incorporate them. Add the flour and mix just until blended—avoid overmixing, as small lumps will dissolve on their own.

Fold the chopped walnuts into the batter, ensuring they are evenly distributed.

Grease a 4x8-inch loaf pan with butter. Line the bottom with parchment paper and butter the parchment as well. Pour the batter into the prepared pan.

Bake for about 1 hour, or until a toothpick inserted in the center comes out clean. Let the loaf cool on a wire rack before removing it from the pan. Slice and serve.