Preheat your oven to 180C
In a bowl mash your bananas with a fork, then add eggs, vanilla and butter to the bowl and whisk together until combined.
In a separate bowl mix together the flour, sugar and salt, then add to the wet ingredients and gently fold in with a wooden spoon.
Spoon the mix into silicon moulds or cupcake cases, no higher than three quarters full (as they'll expand when baking).
Add the Nutella to a small bowl and stir vigorously to soften. Then add a small amount into the centre of each muffin gently swirl with a chopstick or skewer.
Bake in the oven for 20-25 mins, checking doneness with a skewer. Allow to cool on a wire rack before enjoying.