Preheat the oven to 350 degrees F.
Pour cake mix into a 13×9 pan and bake according to package directions.
In a medium saucepan, combine the sugar, cornstarch and salt. Add in the milk and cream and stir to combine. Heat the milk to just under a boil.
Take 1/2 cup of the milk and slowly whisk into the egg yolks.
Add the eggs to the pudding and cook for 3 minutes, then remove from the heat. Stir in the vanilla and the butter. Set pudding aside to cool a little.
Poke holes in the cake using wooden spoon. Then, pour the pudding over the top of the cake, guiding the pudding into the holes. Place the cake in the refrigerator for 30 minutes.
Crush the chocolate cookies into crumbs, and stir them into the 8 ounces of cool whip.
Top the cake with the cool whip mixture and return it to the refrigerator for another 30 minutes.
Crush the remaining 15 Oreo cookies and sprinkle on top of the cake. Garnish with a little chocolate syrup.
Serve this indulgent dessert immediately. Enjoy.