Preheat the oven to 350 F (177 C) for 30 minutes before baking. Lightly grease a 10 inch cast iron skillet.
In a mixing bowl whisk together the flour, baking powder, baking soda and salt until well combined. Set aside for now.
2 cups (240 g) all purpose flour, ½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Slice the cold butter into tablespoon sized slices. Add the butter, light brown and granulated sugar to a microwave safe mixing bowl. Heat the cold butter and sugars in the microwave 30 seconds at a time, mixing in between rounds of heating until the butter is melted and the butter and sugars are well combined. If the butter and sugar mixture is really hot, all the mixture to cool to warm before mixing in the egg.
10 tablespoons (141 g) cold unsalted butter, sliced, ¾ cup (150 g) light brown sugar, ½ cup (100 g) granulated sugar
Mix in the egg, yolk and vanilla extract until smooth and well combined. Mix the dry ingredients into the wet until just combined. I like to do this about ½ of the dry ingredients at a time. Fold in most of the chocolate chips or chunks. Reserve about ½ cup (85 g) for topping the cookie right before baking.
1 large egg, room temperature, 1 large egg yolk, room temperature, 2 teaspoons (10 ml) vanilla extract , 1½ cups (255 g) chocolate chips, divided
Evenly spread the chocolate chip cookie dough into the prepared skillet. Top with the reserved chocolate chips. Bake for 28 - 35 minutes or until the edges of the cookie is pulling slightly away from the edge of the skillet and a toothpick inserted into the center of the cookie comes away with moist crumbs, but not smeared cookie dough.Cool the chocolate chip skillet cookie in the pan on a cooling rack for 20 minutes. Enjoy warm, topped with cold ice cream and a bit of flaky sea salt if you like. Alternatively, scoop or slice the cookie into your preferred number of slices, top with ice cream and enjoy.