Preheat oven to 350°F, line a muffin tin with paper liners, and set aside.
Mix dry ingredients together, including flour, cocoa powder, baking powder, and a pinch of salt.
Blend wet ingredients well, mashing bananas, adding eggs, vanilla extract, and melted butter or oil.
Slowly combine both mixtures, stirring gently until a smooth, lump-free batter forms completely.
Fold in dark chocolate pieces, ensuring even distribution for a rich, sugar-free chocolate taste.
Fill muffin cups evenly, pouring batter until each liner is about two-thirds of the way full.
Bake for 18 minutes, then remove from oven and allow muffins to cool before serving.