1 stick (8 tablespoons) unsalted butter
1/2 cup granulated sugar
1/2 cup light brown sugar
1 teaspoon pure vanilla extract
1 1/2 cups unbleached all-purpose flour
1/2 teaspoon baking soda
1/4 teaspoon kosher salt
1 cup chocolate chunks
Preheat oven to 350 degrees. 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.
Crack an egg onto the butter and sugar mixture, and use a fork to whisk it well 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 minutes, or until starting to turn golden on the top and around the edges, but soft in the center. Serve with vanilla ice cream.