Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.
Combine the chocolate cake mix, water, vegetable oil, and eggs in a large bowl. Beat with an electric mixer on medium speed for 2 minutes until well blended.
Pour the batter into the prepared baking pan and bake for 25 to 30 minutes or until a toothpick comes out clean. Let it cool completely in the pan.
Whisk together the chocolate pudding mix and milk in a medium bowl until smooth. Let it sit for 5 minutes to thicken.
Poke holes all over the cooled cake at 1-inch intervals with the handle of a wooden spoon.
Pour the chocolate pudding mix evenly over the cake, ensuring the holes are filled.
Cover the cake with plastic wrap and refrigerate for at least 2 hours or overnight.
Spread the whipped topping evenly over the cake, then sprinkle crushed chocolate sandwich cookies on top. Decorate as desired.
Notes
Chill thoroughly for best texture and flavor. Keep cake covered in the refrigerator for 3 to 4 days.