Preheat oven to 350℉. Lightly butter a 10-inch cast-iron or ovenproof skillet.
In a medium bowl, whisk flour, baking soda, and salt. Set aside.
In the bowl of an electric mixer, beat the butter, brown sugar, and granulated sugar together until smooth, about 2 minutes.
Add the egg, egg yolk, and vanilla extract. Mix until combined, scraping the bowl as needed.
Add the flour mixture and mix until just combined.
Gently fold in chocolate chips. The dough will be thick and soft.
Spread the batter onto the skillet, pressing it evenly to the edges.
Bake for 22-25 minutes, until edges are golden and the middle is set.
Remove from the oven and immediately sprinkle with flaky salt, if using.
Cool briefly before serving warm with ice cream.
Notes
Use a scale to measure flour. A kitchen scale is essential for consistent results. Too much flour leads to dry and dense cookies. If you don't have a scale, spoon the flour into your measuring cup. Then, level it off with a knife to prevent overpacking.Mini skillet cookies. If you want a skillet cookie that serves 1 to 2, the mini version is perfect. Divide the ingredients in half to make a smaller one. For a 5-inch skillet or smaller, reduce the bake time to 15 minutes.Avoid overbaking. The skillet chocolate chip cookie is ready when the edges are golden brown. While the center might not look quite done, a toothpick will come out clean. If you wait until the middle looks ready, the cookie will be overbaked, dry, and crumbly.Butter the skillet well. A well-seasoned cast-iron skillet doesn't need much butter to prevent the dough from sticking. But the butter is key to getting rich, crispy, golden brown edges.Don't skip the flaky salt. Sprinkle flaky salt once your skillet cookie comes out of the oven. While it's optional, the saltiness balances the sweetness very well. The larger crystals also offer a bit of crunch. It'll add to the flavor without being overpowering.