Preheat the oven to 325 degrees F and grease a 10 or 12-inch cast-iron skillet with vegetable oil and set aside.
In a large bowl or stand mixer fitted with a paddle attachment, cream together the butter and sugars until light and fluffy.
Beat in the eggs and egg yolk one at a time, mixing between each addition.
In a separate, medium bowl, whisk together the flour, baking soda, and salt.
Slowly add the flour mixture to the butter mixture with the mixer running on low speed just until combined.
Fold in the chocolate chips and chopped chocolate.
Spread the cookie dough in the skillet and bake for 30 to 40 minutes.
Allow cooling completely before cutting and serving or scoop out while warm. Serve with ice cream, if desired.