Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it lightly. In a large mixing bowl, mash the ripe bananas with a fork or potato masher until smooth. Add melted butter, vanilla extract, and granulated sugar to the mashed bananas. Mix well until the ingredients are combined.
Beat in the egg until the mixture is smooth and well-blended. In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and ground cinnamon. Gradually add the dry ingredients to the banana mixture, stirring until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the chopped nuts (and chocolate chips or raisins if desired) into the batter. Scoop the batter into the prepared muffin cups, filling each about two-thirds full.
Bake in the preheated oven for approximately 18-20 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.