Prep: Preheat oven to 325°F. Lightly spray a 10-inch cast iron skillet with baking spray, set aside. If you don't have a skillet, use a 9-inch springform pan, cake pan or deep dish pie plate, noting you will have to increase the baking time.
Brown Butter: Melt the sliced butter in a light-colored skillet over medium heat, stirring constantly. Continue to stir, while the butter begins to foam, then continues to sizzle for about 5 minutes, until the milk solids at the bottom of the pan turn toasty brown. It should smell intensely nutty.
Add Sugars: Immediately remove the butter from heat and pour into a large mixing bowl. Add the sugars and whisk to combine. Set aside to slightly cool before adding the eggs.
Add Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt.
Add Eggs: To the butter/sugar, whisk in the eggs, egg yolk, and vanilla until smooth. Using a rubber spatula, gradually stir in the Dry Ingredients just until combined.
Add Chocolate: Stir in the chocolate chips and chunks, reserving a couple tablespoons to sprinkle over the top (purely aesthetic).
Add to Skillet: Transfer the dough to the skillet and smooth into an even layer. Evenly dot with reserved chocolate chips/chunks. Sprinkle with about 1/8 teaspoon coarse sea salt.
Bake: Bake for 30 to 40 minutes, until lightly browned on top and puffed around the edges. Bake closer to 30 minutes for a more gooey center and closer to 40 minutes for more cooked through. If you dig in and the pizookie is too gooey for your liking, pop it back in the oven.
Serve: Let cool for 5 minutes; the center will sink as it cools. Scoop servings onto individual plates or serve in the skillet with spoons. Top with vanilla ice cream and chocolate sauce.