1. Preheat and Prepare:
First, position a rack in the center of your oven and preheat it to 350°F (175°C). There’s no need to grease your cast-iron skillet if it’s well-seasoned. If you’re using a newer skillet or are unsure of its seasoning, you can lightly grease it with a bit of butter or a neutral oil.

2. Mix the Wet Ingredients:
In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Whisk them together vigorously for about a minute. The mixture should be smooth and slightly lightened in color. Add the two room-temperature eggs and the vanilla extract. Whisk again until everything is fully combined and the mixture is glossy. Using room-temperature eggs is important as they will incorporate more easily into the batter, creating a more uniform texture.

3. Combine the Dry Ingredients:
In a separate, medium-sized bowl, whisk together the all-purpose flour, baking soda, and salt. Whisking these dry ingredients together first ensures that the baking soda and salt are evenly distributed throughout the flour, which is crucial for a consistent rise and flavor.

4. Create the Cookie Dough:
Pour the dry ingredient mixture into the wet ingredient mixture. Using a rubber spatula or wooden spoon, gently fold the ingredients together. Mix only until the last streaks of flour have just disappeared. It is vital not to overmix at this stage. Overmixing develops the gluten in the flour, which can result in a tough, dense cookie instead of a soft and chewy one.

5. Add the Chocolate:
Gently fold in your 1 ½ cups of chocolate chips. I like to reserve a small handful to press into the top of the dough before baking for that picture-perfect, chocolate-studded look. You can use semi-sweet chips, milk chocolate, dark chocolate, or even a combination of all three. Chocolate chunks also work wonderfully here.

6. Assemble in the Skillet:
Scrape the cookie dough into your 10-inch cast-iron skillet. Using your spatula or your fingertips, spread the dough evenly across the bottom of the skillet, pushing it all the way to the edges. If you reserved some chocolate chips, press them into the top of the dough now.

7. Bake to Golden Perfection:
Place the skillet on the center rack of your preheated oven. Bake for 25-30 minutes. The baking time can vary slightly depending on your oven. The cookie is done when the edges are golden brown and set, and the center is still slightly soft and looks a little underdone. It will have a slight jiggle in the very middle. This is the secret to a gooey center.

8. Rest and Finish:
Carefully remove the skillet from the oven—remember the handle is extremely hot! Place it on a wire cooling rack. If using, immediately sprinkle the top of the hot cookie with a pinch of flaky sea salt. The salt cuts through the sweetness and enhances the deep chocolate flavor. Let the skillet cookie rest for at least 10-15 minutes before serving. This allows it to set up and continue cooking from the residual heat of the cast iron, ensuring the center is perfectly cooked but still molten.