Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan.
In a large mixing bowl, mash the ripe bananas using a fork or potato masher until smooth, about 2-3 minutes.
Add the melted unsalted butter to the mashed bananas and mix thoroughly until combined.
Stir in the beaten egg and vanilla extract until well combined.
In a separate bowl, whisk together the baking soda, salt, granulated sugar, and all-purpose flour.
Gradually add the dry ingredients to the wet banana mixture, stirring gently until just combined.
Fold in the chopped walnuts, ensuring they are evenly distributed throughout the batter.
In a small bowl, mix together the brown sugar, ground cinnamon, and additional chopped walnuts for the cinnamon swirl.
Pour half of the banana bread batter into the prepared loaf pan and smooth the top.
Sprinkle the cinnamon swirl mixture evenly over the batter in the pan, then pour the remaining banana batter on top.
Use a knife or skewer to gently swirl the cinnamon mixture into the batter for a marbled effect.
Bake in the preheated oven for 50-60 minutes until the edges are golden brown and a toothpick inserted into the center comes out clean.
Allow the banana nut bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.