Preheat the oven to 350°F. Lightly grease an 8 inch cast iron skillet with butter or nonstick spray and set aside.
In a medium bowl, cream the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2 to 3 minutes.
Add the egg and vanilla extract and mix until smooth and fully combined.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing just until no dry streaks remain.
Fold in the semisweet chocolate chips and the dark chocolate chunks, if using, until evenly distributed.
Transfer the cookie dough to the prepared skillet and spread it into an even layer, smoothing the top with a spatula.
Bake for 20 to 25 minutes, until the edges are golden brown and the center is just set. For a gooier cookie, bake closer to 20 minutes.
Remove the skillet from the oven and let the cookie cool for about 5 minutes so it firms up slightly.
Serve warm straight from the skillet, topped with vanilla ice cream if desired.
