Preheat the oven according to the directions on the cake mix package. Grease a 9 x 13-inch baking pan.
Prepare the chocolate cake batter according to the package directions. Pour the batter into the prepared pan and bake until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Let the cake cool for 10 to 15 minutes. While it is still warm, use the handle of a wooden spoon to poke evenly spaced holes across the cake.
Whisk the instant vanilla pudding mix with 2 cups of cold milk for 1 to 2 minutes, until smooth and slightly thickened.
Slowly pour the pudding over the cake, spreading it evenly so it settles into the holes.
Cover and refrigerate for at least 2 hours, or until the pudding is firm and the cake is thoroughly chilled.
Spread the thawed whipped topping evenly over the chilled cake.
Crush the chocolate sandwich cookies and sprinkle them generously over the whipped topping
Garnish with whole cookies or additional crumbs if desired. Refrigerate until ready to serve.