Preheat oven to 170 degrees celsius (fan-forced).
Insert the butterfly into the Thermomix bowl and add the room temperature butter, caster sugar and brown sugar. Mix on Speed 4, 20 seconds.
Scrape down the sides of the bowl and mix on Speed 4, 40 seconds (or until pale and creamy).
Reduce to Speed 2, and add the eggs and vanilla extract through the MC hole. Increase to Speed 4, 15 seconds. Remove the butterfly.
Add the salt, bi-carbonate soda and plain flour and mix on Speed 4, 10 seconds. Scrape down the sides of the bowl and mix for a further 5 seconds.
Mix the chocolate chips through for 10 seconds, REVERSE, Speed 2.
Roll heaped teaspoon-sized balls of the mixture and place onto 2 flat baking trays. Leave a gap to allow for spreading.
Cook for 8-10 minutes or until just lightly golden.
Allow to cool on the baking trays for 10 minutes before transferring to a wire rack to cool completely.