Preheat oven to 350°F (175°C) and prepare a muffin pan with paper liners or grease with nonstick spray.
In a large bowl, mash the bananas until smooth, then whisk in granulated sugar, brown sugar, melted butter, eggs, and vanilla extract until combined.
In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.
Fold the dry ingredients into the wet mixture gently until just combined, avoiding overmixing.
Spoon the batter evenly into muffin cups, filling each about ¾ full.
Add a teaspoon of warmed Nutella on top of each muffin and swirl it gently into the batter using a toothpick or skewer.
Bake for 18 to 22 minutes until a toothpick inserted in the center comes out clean or with minimal Nutella residue.
Cool muffins in the pan for 5 minutes, then transfer to a wire rack to cool further or serve warm.