Combine cold butter and sugar in a large bowl or stand mixer bowl. Beat for 5 minutes.
Add eggs one by one beating to incorporate in between.
Fold in both flours, cornflour, bicarbonate of soda and salt.
Add chocolate and walnuts and fold in.
Form 8 cookies and add 4 to each tray. Bake for 10 - 14 minutes or until golden brown.
Notes
Top tips to make the best Levain Bakery Cookies
Cold Butter - This recipe calls for cold butter which is very unusual in a cookie recipe but this is to stop them from spreading.
The bigger the better! They look abnormally huge but that is exactly how they are supposed to be.
Cool on the tray –Usually you would cool cookies for 5 - 10 minutes and then remove to a cooling rack but that isn't necessary here.
How long do they keep
These cookies keep well in an airtight container for up to a week.
How to freeze
These freeze really well. Once cooled, add to a ziplock bag and freeze for up to 3 months. To serve, let them come to room temperature undisturbed and then place in the oven to warm through for 4 - 5 minutes.