Preheat your oven to 350°F (175°C). Lightly grease an 8-inch (20 cm) cast iron skillet with butter or nonstick spray.

In a medium bowl, use a hand or stand mixer to cream the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Beat in the egg and vanilla extract until fully combined.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix.

Fold in the semi-sweet chocolate chips and dark chocolate chunks (if using).

Transfer the cookie dough to the prepared skillet and spread it evenly with a spatula, smoothing the top.

Bake in the preheated oven for 20-25 minutes, or until the edges are golden brown and the center is just set. For a gooier cookie, bake for less time; for a firmer cookie, bake slightly longer.

Remove the skillet from the oven and let the cookie cool for about 5 minutes. The skillet will continue to cook the cookie slightly as it cools.

Serve warm, directly from the skillet, with scoops of vanilla ice cream on top if desired. Enjoy!