Preheat your oven to 350°F (175°C). Prepare a muffin tin by lining it with paper liners or greasing it.

Mash the ripe bananas in a large bowl until smooth. Stir in the melted butter until well combined.

Incorporate the baking soda, salt, and sugar into the mixture. Then add the beaten egg and vanilla extract, mixing until smooth.

Gently fold in the all-purpose flour until there are no more streaks of flour visible. Do not overmix.

Spoon half of the muffin batter into each muffin tin cavity, then add Nutella into each well.

Gently swirl the Nutella into the batter using a toothpick or knife to achieve a marbled effect.

Bake for 18-20 minutes, or until a toothpick comes out clean. The tops should be golden.

Let the muffins cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely.