Preheat oven to 325ºF. Spread butter into a 10-inch skillet. Set aside.
Whisk flour, ¾ teaspoon of salt, baking powder and xanthan gum in a bowl. Set aside. Using an electric mixer, whisk together melted butter and sugars until well combined. Add in eggs, milk and vanilla until smooth. Stir in flour mixture until batter is completely even. Fold in chocolate chunks. Scrape batter with rubber spatula into skillet. Using a piece of cling wrap, smooth top. Sprinkle the remaining salt.
Bake for 50 to 60 minutes, or until golden brown.
Cool completely in the skillet over a cooling rack.
Recipe by Flavor and Friends at http://flavorandfriends.com/gluten-free-skillet-cookie/