Follow these steps carefully for a perfectly baked, gooey, and delicious Double Chocolate Cookie Skillet. The key is not to overbake!

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Lightly grease a 10-inch cast-iron skillet or an oven-safe skillet of similar size. If you don’t have cast iron, a 9×9 inch baking pan can work, though baking time might vary slightly.
2. Cream Butter and Sugars: In a large mixing bowl, using an electric mixer (stand or handheld), beat the softened unsalted butter, granulated sugar, and packed light brown sugar together on medium speed until light and fluffy. This usually takes about 2-3 minutes. Scrape down the sides of the bowl occasionally. This step incorporates air, which contributes to a lighter texture.
3. Add Eggs and Vanilla: Beat in the large eggs one at a time, mixing well after each addition. Then, stir in the vanilla extract until just combined.
4. Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt. This ensures the leavening and salt are evenly distributed, preventing pockets of bitterness or saltiness.
5. Combine Wet and Dry Mixtures: Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing on low speed until just combined. Be careful not to overmix at this stage; overmixing can lead to a tough cookie. Mix only until you no longer see streaks of flour.
6. Fold in Chocolate: Gently fold in the semi-sweet chocolate chips and the dark chocolate chunks using a spatula or wooden spoon. Ensure they are evenly distributed throughout the dough. The dough will be thick.
7. Press into Skillet: Transfer the cookie dough into the prepared skillet. Using your hands (lightly floured if the dough is sticky) or the back of a spoon, press the dough evenly into the bottom of the skillet. You can sprinkle a few extra chocolate chips or chunks on top for an even more appealing look.
8. Bake: Place the skillet in the preheated oven and bake for 25-35 minutes. The edges should be set and slightly golden brown, but the center should still look slightly underbaked and gooey. A toothpick inserted near the edge should come out with a few moist crumbs, but one inserted in the very center will likely be wet with melted chocolate. For a very gooey center, aim for the lower end of the baking time.
9. Cool Slightly: Carefully remove the skillet from the oven (it will be very hot!) and place it on a wire rack to cool for at least 15-20 minutes before serving. This allows the cookie to set up a bit more and prevents burns from the molten chocolate. The cookie will continue to cook slightly in the hot skillet as it cools.
10. Serve Warm: The best way to enjoy this Double Chocolate Cookie Skillet is warm, perhaps with a scoop of vanilla ice cream or a dollop of whipped cream.