Grease a 10 inch cast iron skillet with cold butter, set aside. On medium heat on the stovetop, heat unsalted butter until completely melted, stirring occasionally. Transfer butter to a large mixing bowl and let sit for a few minutes to cool.
While the butter cools, add flour, cocoa powder, and baking powder to a separate bowl. Whisk together to combine.
Cream together cooled butter with salt, along with granulated and dark brown sugar by beating the mixture with a rubber spatula until it forms a slightly thick, golden-brown paste. Stir in vanilla extract, then beat in the eggs and the egg yolk one by one until completely incorporated into the batter.
Add about 1/3 cup of the dry mixture to the liquid mixture at a time, folding gently with a spatula. Add until dry mixture is finished. Fold until just combined. Do not overmix. Fold in chocolate chips, then spoon the dough into the skillet and smooth with a spatula. Cover with plastic wrap and chill in the refrigerator for at least an hour.
Once dough has chilled, preheat oven to 325 degrees F. Bake for 25-30 minutes, or until the edges are crisp and completely cooked while the center remains just set. The center will look underdone, but the cookie will continue to cook while in the hot skillet. Remove from the oven and let cool on a wire rack completely before serving.