Preheat oven to 350°F. Grease a 12-inch cast iron skillet with butter.
In the bowl of an electric mixer, combine softened butter, brown sugar, and granulated sugar until creamy and smooth. Add the egg, egg yolk, and vanilla; mix until combined.
In a large bowl combine flour, baking soda, and salt. Slowly add dry ingredients to the butter and egg mixture, making sure not to over mix. Spread the batter evenly over the prepared cast iron skillet. Bake for 25-30 minutes or until golden brown. Do not overbake, you want the cookie to be slightly gooey.
Remove from oven and cool. Serve with vanilla ice cream. Enjoy!
