Preheat oven to 170 degrees celsius (fan-forced). Line two large baking trays with greaseproof paper and set aside.
Insert the Butterfly into the Thermomix bowl. Add the softened butter, caster sugar and brown sugar and mix for 20 seconds, Speed 4. Scrape down the sides of the bowl and mix for a further 40 seconds, Speed 4 (or until pale and creamy).
Reduce to Speed 2 and with the butterfly still turning, add the egg and vanilla extract through the MC hole. Increase to Speed 4 for 20 seconds. Remove the butterfly.
Add the salt, bi-carb soda and plain flour to the bowl. Mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Add the chocolate chips and mix for 10 seconds, Reverse, Speed 1 (using the spatula to help mix).
Chill the dough in the fridge for 20 minutes.
Roll into heaped teaspoon sized balls and place onto the prepared trays (leave a 5cm gap between each cookie to allow for spreading).
Cook for 8-10 minutes or until just lightly golden. Do not overcook.
Allow to cool for 10 minutes on the baking trays before transferring to a wire rack to cool completely.