Prepare and bake the cake per the directions on the back of the box.
While the cake is baking, in a small bowl, whisk together the sweetened condensed milk and chocolate syrup until combined well.
Once the cake is out of the oven, while it's still warm, use a fork to poke holes all over the surface of the cake.
Drizzle the condensed milk and chocolate syrup mixture all over the cake. You can use a spatula to help distribute the mixture into the holes, making sure it soaks in rather than running off the sides.
Allow the cake to finish cooling completely to room temperature.
Once the cake has cooled, prepare the topping by whisking together whole milk and pudding mix until it's smooth. Then, whisk in the chocolate syrup.
Gradually add the whipped topping, whisking it in a little at a time until it's fully incorporated. It might have some small lumps, which is okay.
Spread the whipped topping mixture generously over the cake. Don't hold back; use it all.
Cover the cake with plastic wrap and refrigerate it for at least 6 hours to chill and set. If you can, leaving it overnight is even better.
When you're ready to serve, sprinkle crushed Oreos on top.
If you'd like, add a drizzle of chocolate sauce on each slice before serving.
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.
Store leftovers in the fridge for up to a week or freeze for up to 3 months.
The Oreo topping will soften over time so it may be wiser to add to each serving if you don't plan on eating much when you first make it.