Preheat the oven to 350 degrees while you brush the inside of a 8 1/2 x 4 1/2 loaf pan with melted butter.
Whisk together the flour, sugar, baking soda, salt, and walnuts together in a large bowl.
Place the mashed banana, remaining melted butter, yogurt, lemon juice, and vanilla extract into a food processor; pulse until combined well.
Fold the banana mixture into the flour mixture until just combined. Transfer batter to the prepared loaf pan and smooth the top. Bake until the top is firm about 1 hour, rotating pan halfway through.
Let bread cool before serving.