Step 1: Preheat oven to 350°F (175°C). (Optional for a firmer crust; skip for completely no-bake.)
Step 2: Crush chocolate sandwich cookies into fine crumbs. Combine crumbs with melted butter in a bowl until moistened.
Step 3: Press the crumb mixture firmly into the bottom of a 9x13 inch baking dish. Bake for 8-10 minutes if desired, then cool completely. If not baking, refrigerate for 30 minutes.
Step 4: In a large bowl, beat softened cream cheese with powdered sugar until smooth and creamy. Gently fold in 1 1/2 cups of thawed whipped topping until just combined.
Step 5: Spread the cream cheese mixture evenly over the cooled/chilled cookie crust. Refrigerate while preparing the next layer.
Step 6: In a separate large bowl, whisk together the instant chocolate pudding mixes with 3 cups of cold milk for 2-3 minutes, until thickened.
Step 7: Carefully spread the thickened chocolate pudding over the cream cheese layer.
Step 8: Return the dish to the refrigerator for 15-20 minutes to allow the pudding to set further.
Step 9: Spread the remaining 3 cups of thawed whipped topping evenly over the set chocolate pudding layer, creating swirls or peaks.
Step 10: Garnish with chocolate shavings or mini chocolate chips, if desired.
Step 11: Cover loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow all layers to set completely and flavors to meld.