Preheat oven to 400°F (204°C). Line a baking sheet with parchment paper.
In a stand mixer or large bowl, beat cold cubed butter, brown sugar, and granulated sugar on medium-low speed for 1–2 minutes until just combined. The mixture should look thick and slightly chunky, not light and fluffy.
Add eggs one at a time, mixing just until incorporated.
In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.
Add dry ingredients to the wet mixture and mix just until combined. Do not overmix.
Fold in chocolate chunks and chopped walnuts until evenly distributed.
Divide dough into 8 large portions (about 5–6 ounces each). Shape into tall, rough mounds. Do not roll into smooth balls and do not flatten.
Place cookies on prepared baking sheet, spacing them well apart.
Bake for 10–12 minutes, until the edges are golden brown and the tops are lightly browned. The centers should look slightly underbaked.
Allow cookies to rest on the baking sheet for 10–15 minutes before serving. This helps the centers set while staying soft and gooey.