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 egg and vanilla extract through the MC hole. Increase to Speed 4, 15 seconds. Remove the butterfly.
Add the salt, bi-carbonate soda, plain flour and cocoa powder and mix on Speed 4, 10 seconds. Scrape down the sides of the bowl and mix for a further 5 seconds.
Mix the white 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.
Notes
RECIPE NOTES
Insert the Butterfly BEFORE adding the butter and sugars.
Ensure your butter is at room temperature and chopped so that it doesn't get stuck in the Butterfly.
Once you've added the chocolate chips, mix on REVERSE and use the spatula to help combine.
Chilling the dough is optional but HIGHLY recommended. This helps the butter to re-solidify which stops the cookies from overspreading.
The white chocolate chips can be replaced with milk or dark chocolate chips, Reeses peanut butter chips, Caramilk chips or M&Ms.
Freeze the baked cookies or the unbaked dough for up to 3 months.
Store the biscuits in an airtight container at room temperature for up to 5 days.