Add the cubed butter to a medium saucepan over medium heat. Allow the butter to melt, stirring frequently. It will begin to foam and then develop golden brown bits at the bottom of the pan with a nutty aroma. Once browned, remove from heat and allow the butter to cool for 10–15 minutes.

Preheat your oven to 350°F (175°C). Lightly grease a 10–12 inch cast iron skillet or other oven-safe skillet.

Pour the slightly cooled brown butter into a large mixing bowl. Add the brown sugar and granulated sugar. Whisk together until smooth and glossy.

Whisk in the eggs, one at a time, until fully incorporated. Stir in the vanilla extract.

Add the room temperature sourdough discard and mix until fully combined.

Gently fold in the chocolate chips and chopped semi-sweet chocolate until evenly distributed throughout the dough.

Spread the cookie dough evenly into the prepared skillet. For a bakery-style look, press a few extra chocolate chips or chocolate pieces into the top of the dough.

Bake for 25–30 minutes, or until the edges are golden brown and the center is set but still slightly soft.

Allow the skillet cookie to cool in the pan for 10–15 minutes before slicing. Serve warm for the best gooey texture.

The cookie may still appear gooey in the center, but it is done as long as the internal temperature reaches 160–165°F. At this temperature, the eggs and flour are fully cooked and safe to eat, while the center stays soft and fudgy.