Beat the cold butter, brown sugar, and granulated sugar on medium until the mixture looks like coarse sand, about 2 minutes. Add the cold eggs one at a time with the vanilla, mixing just until combined.

Whisk together the flour, cornstarch, baking soda, and salt. Add the dry mixture to the butter mixture in two additions, mixing on low just until no dry streaks remain.

Fold in the chocolate chips and walnuts with a spatula until evenly distributed.

Shape the dough into 6-ounce balls (8 total), keep the surfaces craggy, and place on a parchment-lined pan. Chill for at least 2 hours.

Preheat oven to 400°F (200°C). Place up to 4 dough balls on a parchment-lined baking sheet. Bake for 12 to 14 minutes, until edges are golden and centers look soft.

Cool on the sheet for 10 minutes, then transfer to a wire rack. Serve warm.