Preheat oven to 350 degrees. Line a muffin pan with liners or lightly grease and set aside.
In a large bowl, add peanut butter, bananas, eggs, and maple syrup. Mix until well combined and no (or few) banana lumps remain.
Add cocoa powder and baking soda and mix until well combined.
Pour batter into muffin pans, filling each about ⅔ full. Bake for 20-22 minutes or until a toothpick comes out clean. Remove from oven and allow to stand for 5 minutes transferring to a wire rack to finish cooling.
Maple syrup can be substituted with honey or brown sugar.
To reduce lumps of banana in muffin batter, use a blender to mix ingredients instead.