Preheat your oven to 350°F (175°C) and lightly grease a 9x5 inch loaf pan with butter or cooking spray.
In a large mixing bowl, combine the thoroughly mashed bananas with the melted butter. Stir well until you've formed a smooth and creamy mixture.
Next, mix in the baking soda and a pinch of salt, ensuring they are evenly distributed. Add the sugar, followed by the beaten egg and vanilla extract, stirring until all the ingredients are fully combined.
Gradually add the all-purpose flour to the mixture, carefully folding it in with a spatula until just combined to avoid over-mixing.
In a separate small bowl, whisk together the ground cinnamon and packed brown sugar to create the cinnamon swirl mixture.
Pour half of the banana batter into your prepared loaf pan, spreading it evenly. Then, sprinkle about half of the cinnamon-brown sugar mixture over the top.
Gently pour the remaining banana batter on top, followed by the rest of the cinnamon-brown sugar mixture. Use a knife or skewer to gently swirl the mixture into the batter.
Place your loaf pan in the preheated oven and bake for 60-65 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Once baked, let the banana bread cool in the pan for about 10 minutes. Then carefully transfer it to a wire rack to cool completely before slicing.
Notes
For an inviting serving, slice the banana bread while warm and top each slice with a pat of butter or a dollop of cream cheese.