Preheat oven to 375° F.
In an electric mixer fitted with the paddle attachment, beat the butter, brown sugar, and granulated sugar on medium-high speed for 3 minutes, until light and fluffy. Scrape down the bowl with a rubber spatula. On low speed, add the eggs one at a time, and then the vanilla. Mix until well combined.
Combine the flour, baking soda, and kosher salt in a small bowl, and with the mixer on low, add it to the butter mixture and mix until just incorporated. Using a wooden spoon, gently fold in the chocolate chunks and walnuts.
Sprinkle a large pinch of sea salt over the bottom the bottom of an un-greased 10-inch cast iron skillet. Spoon the cookie batter on top and smooth the surface with a rubber spatula. Sprinkle with another big pinch of sea salt and bake for 30 minutes, until baked through at the edges but still soft in the center. (This will leave the very center of the cookie slightly under-baked. If you prefer a crisper cookie, cook for an additional 5 minutes.)
Serve warm, topped with vanilla ice cream.Note: If serving directly from the oven, use a dish towel or oven mitt to cover the handle of the skillet.Copyright 2015, Lidey Heuck, All Rights Reserved