Begin by preheating your oven to 350°F (175°C). Prepare a muffin tin with paper liners or a light coating of cooking spray.

In a mixing bowl, mash the ripe bananas thoroughly using a fork until smooth with small lumps.

Mix the melted unsalted butter into the mashed bananas. Stir in baking soda, salt, and sugar. Add the beaten egg and vanilla extract, stirring until smooth.

Gently fold in the all-purpose flour until just combined. A few flour streaks are okay.

Spoon half of the muffin batter into muffin cups, creating a small well in the center for Nutella.

Place a teaspoon of Nutella into each well. Aim for a generous amount but don’t overflow.

Cover the Nutella with the remaining batter, filling the cups about three-quarters full.

Using a toothpick or knife, swirl the Nutella into the top layer of the batter for a marbled effect.

Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.

Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.