Cream (3 min): Beat the cold butter with both sugars on medium just until combined and creamy, about 2 minutes. Don't make it fluffy.
Eggs (1 min): Beat in the eggs one at a time with the vanilla, just until mixed.
Dry (2 min): Mix both flours, cornstarch, soda and salt and add on low until barely combined. Stir in chocolate and walnuts by hand.
Portion and chill (40 min): Weigh into 8 balls of about 170 g (6 oz). Keep them tall and craggy, not smooth. Chill at least 30 minutes, or freeze for later.
Bake (12 to 14 min): Heat the oven to 205C (400F). Bake 4 per lined tray, far apart, for 12 to 14 minutes, until golden brown on the outside but still soft and pale in the very middle.
Rest (15 min): Leave them on the tray 15 minutes; they finish setting there.
Tips
Cold butter, cold eggs and a short mix are what keep them tall.