In a stand mixer, combine the cold, cubed unsalted butter and packed brown sugar. Mix on medium speed until light and fluffy, about 3 to 5 minutes. Scrape down the bowl halfway through.

Add one cold large egg and a splash of vanilla extract to the butter-sugar mixture. Mix on low speed until fully blended.

In a separate bowl, whisk together all-purpose flour, cornstarch, baking soda, and a pinch of salt. Gradually add this dry mixture to the wet ingredients while mixing on low speed.

Gently fold in chocolate chunks and chopped walnuts into the dough with a spatula.

Cover the bowl tightly with plastic wrap and refrigerate the dough for 24 to 72 hours.

Preheat your oven to 400°F (200°C) and line your baking sheets with parchment paper.

Scoop about 1/4 to 1/3 cup of dough for each cookie and roll into balls. Place on prepared baking sheets with enough space for spreading.

Bake for 10 to 14 minutes, until the edges are golden brown and the centers remain soft. Bang the baking sheet on the counter after baking.

Let the cookies cool on the baking sheet for 10 to 15 minutes before transferring to a wire rack to cool completely.