Preheat the oven to 350°F (175°C). Line a muffin tin with 8 paper liners or grease it lightly.
In a large mixing bowl, mash the ripe bananas with a fork until smooth.
Add the eggs, vanilla extract, and natural peanut butter to the mashed bananas. Mix well until fully combined.
Stir in the baking soda, salt, and ground cinnamon until evenly distributed.
If using, fold in the chopped walnuts gently.
Divide the batter evenly among the 8 muffin cups.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.