Place butter, salt and sugars into Thermomix bowl.
Beat for 1 min/sp 5, scraping down occasionally.
Add yolks and vanilla beat again on 1 min/speed 5, again scraping occasionally.
Add baking powder and flours and mix 20 secs/Dough . Scrape down and knead again if necessary.
Add choc chips and mix 15 secs/Dough, helping with spatula through the lid.
Tip out on to plastic wrap or ThermoMat and roll into a sausage shape about 25 - 30cm long.
Place in fridge for 2 hours.
Preheat oven to 180C FF. Line two baking trays with baking paper.
Unwrap and use a sharp serrated knife to saw through the log, cutting into 14 - 16 pieces.
Place half of the cookies on one tray and half on the other and bake for 15 minutes.
Swap trays over and reduce heat to 15C FF.
Bake a further 15 - 20 minutes or until golden.
Leave to cool on trays.