Preheat your oven to 350°F (175°C). Prepare the chocolate cake mix according to the package instructions and pour the batter into a greased 9x13-inch pan. Bake for about 30 minutes, or until a toothpick inserted in the center comes out clean. Once done, remove the cake from the oven and let it cool for 10 minutes before moving on.

Using the handle of a wooden spoon, poke holes across the top of the warm cake. Warm the hot fudge sauce in the microwave until it is pourable, about 30 seconds on high, and drizzle it over the cake, allowing it to soak into the holes. Let the cake cool completely at room temperature, which should take about 30 minutes.

In a mixing bowl, whisk the instant chocolate pudding mix with 3 cups of cold milk until smooth and thickened, which should take about 2 minutes. Immediately pour this mixture over the cooled cake, spreading it evenly. Place the cake in the refrigerator for about 20 minutes to allow the pudding to set slightly.

In another bowl, whisk the remaining instant pudding mix with 2 cups of cold milk until it thickens, approximately 2 minutes as well. Gently fold in the thawed Cool Whip until combined, then mix in ¾ of the crushed Oreos.

Spread the pudding mixture evenly over the set chocolate pudding layer on the cake. Top with the remaining crushed Oreos for a finishing touch. Cover the cake with plastic wrap and refrigerate it for at least 2 hours, allowing the flavors to meld.