Preheat the oven to 350°F (180°C). Lightly butter a 10-inch (25 cm) oven-safe skillet, preferably cast iron.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl using a handheld mixer, beat the butter on medium-high speed until smooth, 20 to 30 seconds.
Add the brown sugar and granulated sugar and beat, still on medium-high speed, until smooth, 2 to 3 minutes.
Add the egg, egg yolk, and vanilla extract and beat on high speed until combined, about 1 minute. Scrape the sides and bottom of the bowl and beat again to incorporate.
Carefully add the dry ingredients to the wet ingredients and mix on low speed until combined. Stir in the chocolate chunks or chips. The dough will be soft and sticky.
☞ TESTER TIP: You can cover and refrigerate the dough for up to 4 days or freeze it for up to 3 months. Let the dough return to room temperature before baking. And good luck not letting the snitches get at it!
Gently press the dough into the prepared skillet. Sprinkle with sea salt.
Bake until the edges are lightly browned and set, 25 to 30 minutes. You want the middle to be slightly underbaked.
Remove the skillet from the oven and let it cool for at least 5 minutes before serving. Top with ice cream and chocolate sauce, if desired, and either slice and serve or just hand everyone a spoon! (You can cover the cookie—whether in its entirety or the leftovers—and refrigerate it in the skillet for up to 4 days. Keep in mind the cookie will be softer the day it comes out of the oven and firmer if you refrigerate it first.)