Preheat oven to 375F/190C. Grease muffin pan cups or line them with liner papers.
In a large bowl mix together flour, baking soda, salt, and sugar.
In a medium bowl, whisk together egg, mashed bananas, and melted butter. Pour the egg mixture into the flour mixture and mix as little as possible, just until combined.
Fill the muffin cups halfway with batter and drop about a teaspoon of Nutella in the center of each. Fill with batter the rest of the way, filling the cups about ¾ full. Sprinkle the tops with chocolate chips if you like. Bake for 15-20 minutes or until a toothpick inserted into the center of a muffin comes out clean. Transfer to a wire rack and let cool for 10 minutes before removing from pan.
Muffins will keep fresh and moist for 3 days at room temperature or in the fridge. Serve them at room temperature.