One-Pan Dark Chocolate Chunk Skillet Cookie

1. Preheat oven to 350 degrees F. Melt butter in an 8 inch cast iron skillet set over medium-low heat. Stir in sugars and vanilla and remove from heat. Let rest until pan is warm, but no longer very hot, about 5 minutes.
2. Crack an egg onto the butter and sugar mixture, and use a fork to whisk it into the mixture. Place flour, baking soda, and salt on top, and very carefully stir into the mixture until smooth and well-mixed. Stir in chocolate chunks. Place in the oven for 15-25 minutes, or until starting to turn golden on the top and around the edges, but still soft in the center. Serve with vanilla ice cream.