Preheat your oven to 350 degrees. Lightly grease a cast iron skillet and set aside. Note: I use a 12-inch skillet to make a crisp/chewy cookie. Use a smaller skillet if you want a thicker, chewier cookie.
In a large bowl, combine butter, granulated sugar, and brown sugar. Beat until light and fluffy. Add in the egg and vanilla and beat until combine.
Split the wet batter into two bowls.
To make the chocolate cookie dough: Add in the listed dry ingredients in to the wet ingredients. Mix to combine. Add in the chocolate and stir to thoroughly combine. Set aside.
To make the regular cookie dough: Add in the listed dry ingredients in to the wet ingredients. Mix to combine. Add in the chocolate and stir to thoroughly combine. Set aside.
To bake: Portion out tablespoons of dough into the skillet, alternating between chocolate dough and regular dough to create a marble pattern. When all the dough is in the skillet, pop it into the preheated oven and bake for 20-25 minutes. Enjoy!