Preheat the oven to 180C (360F).
Add eggs, vanilla, butter and mashed bananas to a mixing bowl and combine.
Mix flour, sugar and salt in a different mixing bowl.
Add flour mixture to the wet mixture, and gently fold in. Mixture should be fully combined but still lumpy.
Pour mixture in to greased muffin tin (or cupcake cases).
Drop a teaspoon of nutella in to the centre of each muffin mixture, and gently swirl with a chopstick or skewer (or anything thin).
Don't overmix as you want to see the nutella swirled, rather than fully-combined.
Put muffins in over for 20 to 25 minutes until fully cooked.