Line a tray with baking paper. Spoon teaspoonfuls of Nutella onto the tray and freeze for 30 mins - an hour, until the blobs of Nutella peel off the paper easily.
Pre-heat the oven to 180 C / 360 F.
Add the butter and sugars to the Thermomix bowl. Mix for 1 minute / speed 4.
Add the eggs, vanilla and Nutella to the bowl. Mix for 30 seconds / speed 4.
Add the flour, bicarbonate of soda, cornflour and salt. Mix for 30 seconds / speed 4.
Use a cookie scoop or tablespoon to portion scoops of the cookie dough. Use your thumb to create a dent in the middle of the scoop. Place a nugget of frozen Nutella into the dent and then fold up the cookie dough around it and roll into a ball. Ensure the Nutella is completely encased with cookie dough.
Place the balls on a baking tray lined with baking paper. They will spread as they cook so I only put 6 cookies per tray.
Bake in the oven for around 10 minutes. Remember that cookies are soft when they first come out of the oven and will firm up as they cool.
Allow to cool on the baking sheet for a few minutes until they are firm enough to transfer to a cooling rack to cool completely.
These cookies are best enjoyed when they are still sightly warm and the centre is nice and gooey.