Preheat oven to 350 degrees F.
Make sure you have a 10 inch cast iron skillet. Do not grease it.
Dry ingredients: In a bowl, whisk the flour, baking soda, and salt until evenly combined. Set aside.
Wet ingredients: Using a stand mixer or a handheld mixer, cream the butter with the granulated sugar and the brown sugar on high speed until light and fluffy, about 2 to 3 minutes. Beat in the egg. Add vanilla extract and mix for a few seconds until evenly combined.
On low speed, mix in the dry ingredients with the wet ingredients until evenly combined and a thick dough dough forms. Do not overmix. Using a rubber spatula, fold in the 1 ¼ cups of semisweet chocolate chips.
Spread dough evenly over the ungreased 10 inch cast iron skillet. Top with the ¼ cup of the dark chocolate chunks. Note: the dough should be about ¾ to 1 inch thick.
Bake for 30-35 minutes in the middle rack, or until the edges and top are golden brown, and the center is set (it may be a little bit jiggly in the center, that’s fine as long as it's not too jiggly). It will puff up and then deflate a little. Note: if you use a 12 inch skillet, you may need to reduce the baking time by 3-5 minutes.
Let it cool for at least 20 minutes to allow the cookie to set. Serve with ice cream and drizzle with chocolate or caramel sauce. Enjoy!