Preheat your oven to 350°F (175°C). It's crucial to get the temperature just right for the best baking results. Line a 9×13-inch pan with parchment paper or spray it with non-stick cooking spray. This will make it easier to remove the cake later and keep it from sticking.
In a large mixing bowl, combine the devil’s food cake mix, 1 box of instant chocolate pudding mix, sour cream, vegetable oil, eggs, vanilla extract, and warm water. Using a stand mixer fitted with a paddle attachment or a hand mixer, beat the mixture on medium speed until everything is well combined, typically about 2 minutes. Make sure to scrape down the sides of the bowl to incorporate all ingredients.
Next, take 10 of the Oreo cookies and coarsely chop them. Gently fold the chopped cookies into the cake batter. This will give the cake delightful bursts of Oreo flavor throughout.
Pour the batter into the prepared pan, using an offset spatula or spoon to spread it into an even layer. Ensure the batter is evenly distributed for consistent baking.
Place the pan in the preheated oven and bake for 30 to 35 minutes. You’ll know it’s done when the top springs back when lightly touched. Keep an eye on it towards the end to avoid overbaking.
Once baked, remove the cake from the oven and allow it to cool in the pan for about 10 minutes. After cooling, use a smoothie straw or the handle of a wooden spoon to poke holes across the surface of the cake, spacing them about one inch apart. This will allow the pudding mixture to seep into the cake, creating that signature poke effect.
In a medium bowl, whisk together the remaining box of instant chocolate pudding mix and whole milk for about 2 minutes until smooth. Immediately pour the pudding evenly over the cake, making sure it fills the holes you created. Let the cake cool completely after this step.
Now, finely chop the remaining 8 Oreo cookies and gently fold them into the whipped topping. This will add flavor and some texture to the topping.
Spread the whipped topping mixture evenly over the cooled cake. This layer should be fluffy and creamy, giving your cake a beautiful finish. Cover the cake and chill it in the fridge for at least 4 hours to allow the flavors to meld.
Just before serving, coarsely chop any leftover Oreo cookies and sprinkle them on top of the cake. If desired, drizzle some chocolate fudge sauce over the top for an extra touch of indulgence. Slice, serve, and enjoy!