In a large bowl, cream the cold, cubed butter with both sugars. Because the butter is cold, it won’t get fluffy; it should look like a thick, grainy paste. This prevents the cookies from spreading too much.
Add the eggs and vanilla, mixing just until they disappear.
Mix in the flour, cornstarch, baking soda, and salt. Mix until you see only a few streaks of white flour left. Do not overmix, or the cookies will become bread-like.
Hand-fold your milk chocolate chunks and semi-sweet chips.
Scoop the dough into large mounds (about 110g to 125g each).
Place the dough balls in the fridge for 2 hours at least.
Preheat your oven to 180°C. Bake for 10–12 minutes. They should look golden on the edges but still appear slightly “wet” or underdone in the center.
Leave them on the baking sheet for at least 15–20 minutes. They will finish cooking with the residual heat.
