Fire up your oven, turn the gas dial to low and preheat for 15 minutes, aiming for 190 °C on the stone baking board inside. Use an infrared thermometer to quickly and accurately check the temperature.

In a large bowl, mix sugar, butter, and vanilla together until no lumps remain. Crack in the egg and stir to combine until the mixture resembles cake batter.

Sift in the flour and baking powder and mix until no flour streaks remain. Add half the chocolate chips and all of the chopped dark chocolate. Mix everything together, combining with your hands if necessary.

Lightly rub 1 teaspoon of oil into your pre-seasoned cast iron skillet pan (or butter, if you prefer a crispier edge). Spread the cookie dough into the skillet, scraping down the sides of the bowl. Use damp hands to gently pat the cookie dough down into the corners of the pan to create an even surface. Add the remaining chocolate chips and sprinkle with flaked sea salt. Set a timer. Turn off the oven and place the skillet inside to cook for 5 minutes. After 4 minutes, rotate the pan 180 degrees and turn on the oven to its lowest setting.

Cook for another 5 minutes. At the 10-minute mark, turn the pan 90 degrees. After two minutes, turn the pan another 180 degrees. From here, turn the pan 90 degrees every minute for another 4 minutes until you reach 16 total minutes of cooking. By this point, the cookie will begin to smell delicious and puff up, turning medium golden brown. Remove the skillet from the oven to a safe surface and insert a toothpick or knife tip into the center; give or take some melted chocolate, the knife should come out clean.

If serving warm, let the cookie rest for 5 minutes before slicing, then top with a scoop of ice cream. If you prefer a more solid cookie, let it reach room temperature, then slice (as it cools, the cookie will harden) and transfer to a container to refrigerate for at least one hour before slicing and serving.