Preheat the oven to 350°F (center rack).

In a medium bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking soda, and 1/2 teaspoon sea salt. Set the bowl aside and reserve a small pinch of the sea salt for topping.

In a large bowl, cream 1 cup unsalted butter (at room temperature) with 1/2 cup dark brown sugar and 1/2 cup white granulated sugar until light and fluffy, about 3 minutes (use an electric mixer on medium speed or beat vigorously by hand).

Add 1 large egg and 1 1/2 teaspoon vanilla extract to the butter-sugar mixture and mix until thoroughly combined.

Add the flour mixture to the butter mixture in two additions, mixing just until no visible streaks of flour remain. Do not overmix.

Fold in 1 1/2 dark chocolate chunks and 1 cup roughly chopped walnuts until evenly distributed throughout the dough.

Transfer the dough to a well-seasoned 12-inch cast-iron skillet and spread it into an even layer. Sprinkle the reserved pinch of sea salt on top if desired.

Bake on the center rack at 350°F for 20 minutes. After 20 minutes, reduce the oven temperature to 300°F and bake an additional 15 to 20 minutes, until the edges are golden brown and the center is still slightly soft when gently jiggled.

Remove the skillet from the oven and allow the cookie to cool completely in the skillet before slicing. Serve cookie slices at room temperature.