Preheat oven to 160 degrees celsius (fan-forced) or 325 degrees fahrenheit. Line two large baking trays with greaseproof paper and set aside.
Place the plain flour, bi-carbonate soda and salt in to the Thermomix bowl. Sift by mixing for 10 seconds, Speed 8. Set the dry mixture aside in a seperate bowl.
Chop the butter and place into the Thermomix bowl. Melt for 3 minutes, 60 degrees, Speed 2. Allow to cool slightly.
Add the caster sugar, brown sugar and vanilla extract. Mix for 20 seconds, Speed 4. Scrape down the sides of the bowl.
Set the machine to Speed 2 and add the egg through the MC hole. Once combined, add the extra egg yolk and continue to mix until combined.
Add the dry mixture to the bowl. Mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Fold the white chocolate chips through with the spatula.
Place the dough into the fridge for 2 hours (this is NOT an optional step - the dough requires chilling).
Roll into tablespoon sized balls and place onto the prepared trays (leave a 6cm gap between each cookie to allow for spreading).
Cook for 10 minutes or until just lightly golden around the edges.
Allow to cool slightly on the baking trays before transferring to a wire rack to cool completely.