Preheat the oven to 350°F.

Spray an 8-inch cast-iron skillet with nonstick cooking spray.

Place the softened butter, brown sugar, granulated sugar, and vanilla extract in a large bowl. Use a mixer to cream together the butter and sugars on medium speed until pale and fluffy, about 1 minute. Add the egg and continue to beat on medium speed until light and fluffy, 30 seconds to one minute.

Add the flour, corn starch, baking soda, and salt to the bowl. Beat the mixture on medium speed until all ingredients are combined, about 1 minute. Scrape down the sides of the bowl with a rubber spatula as needed.

Fold the chocolate chips into the cookie dough until they are evenly distributed.

Transfer the cookie dough to the prepared cast-iron skillet. Use a spoon or clean hands to press the dough into an even layer.

Bake the cookie on middle rack of oven until golden brown at the edges and a toothpick inserted in the center comes out clean, 28-30 minutes.

Check to see that cookie is done. Remove from oven or add time as needed.

Sprinkle flaky sea salt on top of the cookie, if desired. Allow the cookie to cool for 10 minutes before serving. Serve the cookie warm directly from the skillet. Alternately, the cookie may be cooled to room temperature in the skillet, then cut into slices.