Preheat oven to 375 degrees F (190 degrees C).
In an electric mixer fitted with the paddle attachment, beat the Unsalted Butter (1 cup), Brown Sugar (1 cup), and Granulated Sugar (1/2 cup) on medium-high speed for 3 minutes, until light and fluffy. Scrape down the bowl with a rubber spatula.
On low speed, add the Large Egg (2) one at a time, and then the Vanilla Extract (1 teaspoon). Mix until well combined.
Combine the All-Purpose Flour (2 cup), Baking Soda (1 teaspoon), and Kosher Salt (1 teaspoon) in a small bowl, and with the mixer on low, add it to the butter mixture with until incorporated. Using a spoon, fold in the Semi-Sweet Chocolate Chips (8 ounce) and Walnut (2/3 cup).
Sprinkle a large pinch of Sea Salt Flakes (to taste) over 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 pinch of sea salt and bake for 30-35 minutes, until baked through but still slightly soft in the center.
Serve hot with Vanilla Ice Cream (to taste).