Preheat oven and combine butter and sugars: Preheat oven to 350 degrees. In a large bowl, combine butter and sugars with a wooden spoon.
Add egg and vanilla: Stir in egg and vanilla.
Add dry ingredients: Stir in flour, baking soda, and salt.
Add chocolate chips: Stir in chocolate chips.
Transfer to skillet: Transfer cookie dough to a 10-inch cast-iron skillet; smooth top.
Bake and serve: Bake until cookie is golden brown and just set in the center, 18 to 20 minutes. Let cool 5 minutes and serve warm.
