Preheat your oven to 350ºF and grease a 10-inch cast iron pan with butter.
In the bowl of a stand mixer with the paddle attachment, or with an electric hand mixer, cream the butter and sugars together until light and fluffy.
Add in the eggs and the vanilla extract and beat on low-medium speed until fully combined.
In a separate bowl, sift together the flour and cocoa powder, then add the baking soda and salt and mix until combined.
Add the dry ingredients to the wet ingredients in three parts, mixing on low speed until just combined. Don't over-mix! Your dough should be fluffy, not tough, with no streaks of flour left.
Gently fold in the chopped chocolate, saving a few chunks for topping.
Press the cookie dough into your cast iron pan in an even layer and sprinkle the remaining chocolate chunks over the top of the skillet cookie.
Bake for 30-35 minutes, until the sides of the cookie puff up, but the center is still soft.
Let the skillet cookie cool in the pan for about 5-10 minutes depending on how warm you want it when serving. Top with your favorite ice cream and enjoy!