Preheat oven to 170 degrees celsius (fan-forced). Line 3 large flat baking trays with baking paper and set aside.
Insert the butterfly into the Thermomix bowl. Add the butter, brown sugar and caster sugar and mix for 3 minutes, Speed 3 (scraping down the sides occasionally).
Add the eggs and vanilla extract and mix for 1 minute, Speed 3.
Add half of the Nutella (¼ cup) and mix for 30 seconds, Speed 3 (reserve the remaining ¼ cup for swirling through the mixture before baking).
Remove the butterfly. Add the plain flour, bi-carb soda and salt and mix for 30 seconds, Speed 3. Scrape down the sides of the bowl and mix again for an extra 30 seconds.
Sprinkle over ¾ cup of chocolate chips (reserve the remaining ¼ cup) and stir through with the spatula.
Add the remaining Nutella and swirl through with a spoon to make streaks through the mixture.
Roll tablespoon-sized balls of the dough and place onto the prepared trays, leaving a 5cm gap between each dough ball to allow for spreading.
Bake for 10 minutes or until just ever so slightly golden around the edges (don't overcook). Rotate the trays during cooking.
Remove from the oven and immediately gently press 2-3 chocolate chips onto the top of each cookie. Sprinkle with a little sea salt. The cookies will deflate as they cool.
Leave on the trays for 5 minutes and then transfer to a wire rack to cool completely.