Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, combine the mashed bananas, peanut butter, honey, eggs, and vanilla extract. Mix until smooth and well combined.
In another bowl, whisk together the rolled oats, protein powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix.
Fold in the chopped walnuts or pecans and dark chocolate chips if using.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 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 5 minutes before transferring them to a wire rack to cool completely.