Preheat the oven to 350 degrees. Prepare a 13x9 baking pan and spray the bottom with nonstick cooking spray. Set the pan aside.
Make your cake batter according to the directions on the cake mix box. Bake according to the package directions. Place the cake on a cooling rack and let it cool down once baked.
To make the homemade pudding, combine the sugar, cornstarch, salt, milk, and cream in a saucepan. Using a fork, stir the mixture until the cornstarch dissolves.
Heat the milk mixture to just under a boil while occasionally stirring.
When you see tiny bubbles forming on the sides of the pan, it’s time to temper the eggs. To temper the eggs, take 1/2 cup of the milk mixture and slowly whisk it into the egg yolks.
Slowly add the eggs to the pudding mixture and cook for around 3 minutes or until you get the right consistency. Continue stirring. Once you get the right consistency, remove from heat.
After removing from heat, add the vanilla and the sliced butter into the pudding mixture. Stir until butter fully melts and is well combined.
With the handle of a wooden spoon, poke holes through the cake. Space the holes about 1 inch apart.
Pour the pudding over the top of the cake, guiding the pudding into every hole. Make sure to spread the pudding evenly, covering all the holes in the cake.
Place the cake in the refrigerator for at least 30 minutes.
Take a sealable bag and put 5 Oreos inside. Using a wooden roller, crush the chocolate cookies into crumbs. Once you’re done crushing, get a mixing bowl and add the crushed Oreo cookies and 8 ozs of cool whip. Stir until well combined.
Top the cake with the cool whip and cookie mixture. Spread it evenly on the cake using a spatula and return it to the refrigerator for another 30 minutes to let it set.
Just before serving, grab your sealable bag again and crush the remaining 15 Oreo cookies. Sprinkle them on top of the cake evenly.
Top the cake with a bit of chocolate syrup drizzle. It is best when served immediately, right after you pop it out of the fridge.