Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease them well.
In a mixing bowl, combine the mashed bananas, granulated sugar, brown sugar, and vegetable oil. Mix well until smooth.
Add the egg and vanilla extract to the banana mixture, and mix until well incorporated.
In a separate bowl, whisk together the baking soda, salt, and all-purpose flour. Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Avoid overmixing; a few lumps are okay.
Scoop about 1/4 cup of the batter into each muffin cup, filling them about halfway.
Drop a teaspoon of Nutella into the center of each muffin cup and use a toothpick or a knife to swirl it gently into the batter, creating a marbled effect.
If desired, sprinkle chopped hazelnuts or walnuts on top of the muffins for added texture and flavor.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Notes
For added texture, top with chopped hazelnuts or walnuts.