Preheat oven to 180 degrees celsius (fan-forced).
Line a 12 hole muffin tray with muffin liners.
Place bananas into the Thermomix bowl. Mash for 10 seconds, Speed 8. Pour into a bowl and set aside.
Place the butter into the Thermomix bowl (don't worry about cleaning it) and melt for 1 minute, 100 degrees, Speed 2. Scrape down the sides and melt for a further 30 seconds, 100 degrees, Speed 2 (if needed). Allow to cool slightly.
Add the vanilla extract, honey and milk and mix together for 20 seconds, Speed 4. With the blades spinning, reduce to Speed 3 and add the eggs one at a time through the MC hole.
Add the mashed bananas and mix for a further 20 seconds or until well combined.
Add the self-raising flour, caster sugar, baking powder, bi-carb soda, cinnamon and salt and mix for 5 seconds on Speed 4 (use the spatula to help mix).
Divide mixture evenly between the muffin cases. Add 1 heaped teaspoon of Nutella to the centre of each muffin. Swirl around using a skewer or knife. Cook for 15 minutes or until they spring back when lightly touched in the middle. Allow to cool in the tray for 5 minutes before transferring to a wire rack to cool completely.