Preheat oven to 180°C and line at least 3 baking trays with paper, you may need more depending on the size you make the cookies and the size of your oven trays.
Place cold butter, sugars and vanilla into TM bowl and chop 5 sec/speed 5. Whip 2 min/Butterfly/speed 4. Remove Buttefly.
Add eggs, baking powder and cornflour and blend 6 sec/speed 6.
Add about half the flour and combine 8 sec/speed 8. Add remaining flour and combine with the aid of the spatula 10 sec/speed 8.
Finish folding in the flour by hand. It will be very stiff. Toss the chocolate chips and nuts in the tablespoon of flour and fold through the mixture.
Remove the dough from the TM bowl and refrigerate for around 30 minutes.
Divide cookie dough into portions and roll, place onto the trays and flatten slightly with the base of a glass. Bake 15 minutes, no longer, the Levain cookies are slightly undercooked if anything, but they still hold their shape, unless you are carrying them around in a back pack because you have eaten yourself silly whilst in NYC and cannot possibly fit another morsel in. (DId I say that out loud?)
They will keep a brief few days if your household is like ours, but should actually keep at least a week in an airtight container, though they are better on the day of baking.