Preheat your oven to 350°F (175°C). This ensures the oven is hot enough for baking when the cookie dough is ready.
In a large mixing bowl, sift together the all-purpose flour, baking soda, baking powder, and salt. Whisk these dry ingredients until they're well combined and set them aside. This mixture will help your cookie maintain structure and rise correctly.
Heat your 12-inch cast iron skillet over medium heat. Add the unsalted butter and let it melt completely. Continue to cook, stirring continuously, until the butter turns a golden amber color with small brown specks forming at the bottom of the pan. This browning process is crucial as it adds a deep, nutty flavor to your cookies.
Remove the skillet from the heat and immediately stir in both the brown sugar and white sugar into the brown butter until well combined. This creates a rich and flavorful base for your cookie.
Allow the butter and sugar mixture to cool slightly in the skillet. It’s important to ensure that this mixture is not too hot to prevent cooking the eggs in the next step.
Once the mixture has cooled, whisk in the beaten egg and egg yolk until fully incorporated. This adds moisture and binds the dough.
Stir in the sourdough discard and vanilla extract, mixing until everything is well combined. The sourdough discard adds a unique tangy flavor that elevates the cookie from the usual.
Gradually add the dry ingredients to the wet ingredients directly in the skillet, mixing until just combined and no dry spots remain. This method minimizes cleanup and integrates the dough evenly.
Fold in the chocolate chips, and if desired, sprinkle additional chocolate chips on top for extra chocolatey goodness.
Smooth the top of the cookie dough with a spatula, and clean any stray flour from the sides of the skillet with a wet paper towel for a neat presentation.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the edges start to pull away from the skillet. The timing is important to achieve the perfect texture—gooey on the inside and slightly crisp on the outside.
Remove the skillet from the oven and let the cookie rest for a few minutes before serving. Serve it warm, ideally with a scoop of vanilla ice cream or whipped cream on top, for a delicious contrast of hot and cold.
Notes
Butter Browning: Watch the butter closely as it browns; it should be golden with tiny specks, not burnt. This step adds a rich, nutty flavor, crucial for the overall taste.
Mixing Dough: Mix the dough just until combined. Overmixing can lead to a tough texture once baked. Aim for a homogeneous mix without overworking it.
Chocolate Chips: Add the chocolate chips just before baking to prevent them from sinking to the bottom of the skillet. You can also reserve some to sprinkle on top for a more appealing finish.
Baking Time: Keep an eye on the oven as baking times may vary. The cookie should be golden around the edges but still soft in the center. Overbaking can result in a dry cookie.