Preheat your oven to 350°F (175°C). If using muffin liners, place them in a muffin tin or lightly grease the tin.
In a large mixing bowl, mash the ripe bananas with a fork or potato masher until smooth.
Stir in the melted coconut oil (or butter) until well combined.
Add the brown sugar, egg, and vanilla extract to the banana mixture, whisking until smooth and evenly mixed.
In another bowl, combine the baking soda, salt, ground cinnamon, and all-purpose flour. Mix well.
Gradually fold the dry ingredients into the wet mixture using a rubber spatula. Be careful not to overmix; it's okay if there are a few lumps.
If desired, gently fold in the chopped walnuts and chocolate chips.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove from the oven and allow the muffins to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely.