Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or lightly grease each cup to prevent sticking.
In a large mixing bowl, mash the ripe bananas with a fork or potato masher until smooth but still a little chunky for texture.
Add the melted butter, beaten egg, vanilla extract, and sugar to the mashed bananas. Stir well until all ingredients are fully incorporated and the mixture is smooth and creamy.
In a separate bowl, whisk together the flour, baking soda, and salt to distribute the leavening agent evenly.
Gradually add the dry ingredients to the wet banana mixture. Stir gently with a spoon or spatula until just combined. Be careful not to overmix.
Using a spoon or small scoop, fill each muffin cup halfway with batter. Add about a teaspoon of Nutella in the center of each muffin. Cover the Nutella with more banana batter until the muffin cups are about 3/4 full.
Bake the muffins in the preheated oven for 18-22 minutes until golden and a toothpick inserted (avoiding Nutella center) comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack. Enjoy warm or at room temperature.