In a large bowl, beat the cold, cubed butter with both sugars until creamy and just combined.
Add in the egg, egg yolk, and vanilla extract. Mix just until combined.
In a separate bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt.
Add the dry ingredients to the wet mixture and fold until just combined.
Fold in chopped chocolate and walnuts evenly.
Divide the dough into 100g portions, shape them into tall mounds, and place on a tray lined with parchment paper.
Freeze for at least 4–6 hours (best overnight) until solid.
When ready to bake, preheat your oven to 200°C.
Place frozen dough balls spaced apart on a lined baking tray and bake for 10–12 minutes, until the edges are golden and the centers still look slightly underbaked.
Let cookies rest on the tray for 5–10 minutes before transferring. Serve warm!
