Preheat oven to 170 degrees celsius (fan-forced). Line two large baking trays with greaseproof paper and set aside.
Place the softened butter, caster sugar and brown sugar into the Thermomix bowl (with the butterfly inserted). Mix on Speed 4 for 20 seconds. Scrape down the sides of the bowl and mix for a further 40 seconds, Speed 6 (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 on Speed 4, 10 seconds. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Fold the chocolate chips through with the spatula.
Place the dough into the fridge for 15 minutes (optional).
Roll into heaped teaspoon sized balls and place onto the prepared trays (leave a gap between each cookie to allow for spreading).
Cook for 8-10 minutes or until just lightly golden.
Allow to cool slightly on the baking trays before transferring to a wire rack to cool completely.