Preheat the oven to 180°C/350°F (160°C fan) and line a 12 hole tin with muffin cases.
Melt the butter in the Thermomix bowl for 40 sec/50℃/speed 4. (Omit this step if the butter is already at room temperature).
Add all the other ingredients to the Thermomix bowl and mix for 5 sec/speed 4. Scrape down the bowl and gently mix the rest with a spatula until fully combined. (try not to over mix otherwise the muffins will become dense). It doesn't matter if the mixture is lumpy.
Spoon the mixture into the muffin cases until 2 thirds of the way up.
Blob some Nutella mixture on top of each muffin and gently swirl around with a toothpick.
Bake in the oven for 20-25 minutes (depending on the size) until lightly golden and a toothpick inserted comes out clean.
Eat warm straight from the oven and enjoy!
Notes
Storage: The muffins taste best fresh from the oven but they can be stored in an airtight container for a few days or in the freezer for up to 3 months.