Bake the cake according to the instructions on the box. Use a 9x13 baking pan. Once the cake is finished baking, let it cool off for about 10 minutes.
While the cake is still warm, you're going to want to poke holes across the top of it using the back of a wooden spoon. Don't go all the way through though, about halfway is plenty.
Microwave the hot fudge sauce so that it pours easily. Usually you can get the right consistency after popping it in the microwave for about 30-60 seconds. Once it's warm enough, go ahead and drizzle it over the holes you made, ensuring that it gets down in there nicely.
When you're done, let the cake cool off completely.
In a medium-sized bowl, dump out one box of the instant dark chocolate pudding mix and whisk it with the cold milk. After it's mixed but before it sets, spread it over the top of the cooled cake.
Pop it in the fridge for about 20 minutes.
Take the cake out of the refrigerator and sprinkle about 2/3 of the crushed Oreos evenly over the pudding layer.
In another bowl, mix the remaining box of dark chocolate pudding with the rest of the cold milk. Once it starts to thicken, gently fold in the whipped topping. Spread the combined mix over the Oreo layer.
Sprinkle the remaining crushed Oreos on top. If you're using the dark chocolate candy bar, scatter the chopped pieces over the dessert.
Refrigerate the dessert for at least 4 hours. Ideally you should leave it in there overnight to allow all the yummy flavors to completely combine. Slice, serve, and watch it disappear!