In a large mixing bowl, combine the two packets of instant chocolate pudding with 3 cups of cold milk. Whisk the mixture until it thickens, about 2 minutes.
Gently fold in the thawed whipped topping into the pudding until well combined.
Place a layer of graham crackers at the bottom of your 9x13 inch baking dish. Spread half of the chocolate pudding mixture over the graham crackers.
Sprinkle half of the diced Snickers and halved Reese's Peanut Butter Cup Minis over the pudding layer.
Add another layer of graham crackers followed by the remaining chocolate pudding mixture. Top with the rest of the Snickers and Reese's Peanut Butter Cups.
Add a final layer of graham crackers on top. Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight.
Once chilled, cut the Chocolate Candy Bar Icebox Cake into squares and serve.