Heat the skillet over medium-low heat. Add butter and allow to melt. Once the butter is melted, start stirring the butter constantly, watching it carefully. The butter will foam, then turn golden, then start to brown and develop small brown flecks. Let it brown and remove from heat.
Pour the butter into a large mixing bowl. Add the sugars and stir to incorporate fully. Let cool just slightly.
Stir in the egg and vanilla, then add the flour, baking soda and salt and stir until a batter forms.
Add the chocolate chips and incorporate fully.
Pour the batter into the skillet and slide the skillet into the oven. Bake for 15-18 minutes until the cookie is set. Let cool on an iron rack for about 5 minutes, then cut into wedges and enjoy!
Recipe by Grits and Chopsticks at https://www.gritsandchopsticks.com/the-gravy-train/2016/04/salted-cast-iron-chocolate-chip-cookie.html