Preheat the oven to 350°F (175°C). Grease a 9×5 inch loaf pan and set aside.
In a large mixing bowl, combine the mashed bananas and melted butter. Mix well.
Add the granulated sugar, eggs, and vanilla extract to the banana mixture. Stir until fully combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, ground nutmeg, and ground allspice.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
If using, fold in the chopped walnuts.
Pour the batter into the prepared loaf pan and spread it evenly.
Bake in the preheated oven for 60 minutes or until a toothpick inserted into the center comes out clean.
Remove the banana bread from the oven and allow it to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing and serving.