Preheat oven to 350°F (175°C). Line or grease a muffin tin.
Mash bananas in a large bowl until smooth.
Stir in melted butter. Add baking soda, salt, and sugar and mix well. Stir in egg and vanilla.
Gently fold in flour until just combined. Do not overmix.
Spoon half the batter into muffin cups. Add a teaspoon of Nutella to each, then top with remaining batter.
Swirl Nutella gently with a toothpick. Bake 18–20 minutes, until a toothpick comes out clean.
Cool in pan for 5 minutes, then transfer to a wire rack.