Preheat oven to 350 degrees F. Add butter to a large bowl and melt in the microwave. Add chocolate chips and stir to melt. If they don't fully melt, microwave for another 15 seconds and stir again. Let cool for 5 minutes.
Whisk in the granulate sugar, brown sugar and vanilla. Add eggs, one at a time, whisk well between additions. Sprinkle in flour, cocoa powder and salt. Fold until just combine and you can no longer see any flour.
Grease a 10-inch cast iron skillet lightly with shortening or vegetable oil. Pour batter into prepared skillet. Bake for 25 to 30 minutes. Check with a toothpick, it should have slightly wet crumbs.