Heat the Nutella and mix it with the cocoa powder. Pour the chocolate mixture into a clean, empty plastic tube and freeze overnight.
Beat the butter, sugar, brown sugar, and salt with a hand mixer until creamy. Add the flour and knead until smooth. Wrap the cookie dough in plastic wrap and refrigerate overnight.
Cut off the bottom of the plastic tube, push out the frozen chocolate log, and slice it into several evenly-sized pieces. Place a 10-inch heat-resistant pan lid upside-down on a bowl. Grease the lid and spread out the chocolate chips on it.
Roll 2/3 of the chilled cookie dough out into a circle, carefully press it into the pan lid over the chocolate chips, and cut off any excess dough.
Spread vanilla pudding over the cookie dough.
Combine the ingredients for the chocolate sauce and spread it over the vanilla pudding.
Spread chocolate pudding over the chocolate sauce.
Roll the rest of the cookie dough out into a circle, place it on top of the chocolate pudding, and use a fork to seal the 2 cookie dough parts.
Flip the lid over into a large cast iron skillet. Transfer the skillet with the lid to a preheated oven and cook at 350°F for 1½ hours. Carefully remove the finished cookie from the lid and let it cool down completely before serving, preferably overnight.