Preheat oven to 350.: The moment you begin, you will notice the small hum and warmth as the oven comes up to temperature, which creates consistent rise and crumb in the chocolate cake . Preheating ensures the cake bakes evenly, avoiding a dry edge. A common mistake is placing the pan in too soon, which leads to uneven texture, so wait until the oven displays the correct temperature.
Make cake as directed on package using a 9×13 pan.: As the batter goes into the pan, you should see a glossy pour that settles smoothly; this shows proper ratio of wet to dry. Spreading it evenly ensures uniform baking and consistent pockets for the pudding. Avoid overmixing, which develops gluten and yields a tougher crumb.
While cake is still semi-warm, poke holes into your cake about 1-inch apart. I used the end of wooden spoon.: You will feel slight resistance as the spoon tip goes through the crumb, leaving neat holes that act like wells for the pudding. Doing this while semi-warm helps the cake accept the filling without collapsing. A mistake to avoid is waiting until it is fully cooled, which makes holes harder to create and reduces pudding penetration.
Make pudding by mixing together the pudding mix and milk until smooth. Pour over cake making sure it falls into the holes. Refrigerate cake for at least one hour.: Whisking vigorously produces a glossy, thickened pudding that smells sweet and creamy. As you pour, listen for a soft splashing as the pudding fills pockets, and watch it sink slowly, leaving even coverage. Chilling firms the filling and sets the texture, so skip the refrigeration step only if serving immediately. Stirring inadequately can leave lumps, so whisk until perfectly smooth.
Top with whipped cream and crushed Oreos. Serve immediately or refrigerate until ready to serve.: The final assembly brings cool, airy whipped cream on top with crunchy cookie bits that crackle against the fork. The contrast between chilled pudding and whipped topping is pleasing on the palate. If you refrigerate after topping, the whipped cream may settle slightly, so add extra cookie crunch just before serving. Avoid crushing the Oreos into dust unless you want a uniform coating rather than distinct chunks.
