Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or lightly grease it.
In a large mixing bowl, combine the mashed bananas and melted coconut oil. Stir until well mixed.
Add the brown sugar, beaten egg, and vanilla extract to the banana mixture. Whisk until everything is incorporated.
Sprinkle the baking soda and salt over the mixture, then add the ground cinnamon. Mix again to combine.
Gradually fold in the all-purpose flour until just combined; be careful not to overmix.
Gently fold in the chocolate chips, and if using, the chopped walnuts or pecans.
Spoon the batter evenly into the prepared muffin tin, filling each cup about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Arrange the muffins on a wooden board, dust with a little powdered sugar, and place a few banana slices and chocolate chips on top for a decorative touch.