Preheat the oven to 350°F. Prepare a 9×13 inch baking pan with a light coating of cooking spray. Set aside.
In a large bowl of a stand-up mixer (you can use a hand mixer also), combine cake mix, water, oil, and eggs. Mix until thoroughly combined.
Pour batter into the prepared baking pan and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Remove from oven and let cool for about 10 minutes.
While the cake is still warm, poke holes all over the cake using the end of a wooden spoon, chopstick, skewer, or anything that you have that will give a round hole to the cake. Make sure the cake is poked evenly all over the cake so that you will get an even application with the chocolate sauce.
Place the condensed milk, chocolate chips, and Nutella in a microwave-safe bowl. Microwave in 20-second intervals, whisking until smooth and combined. Reserve about 1/3 cup of chocolate sauce for drizzling over the frosting.
Pour the chocolate filling all over the cake. Set aside to cool. Then let the cake chill in the fridge for an hour until completely cooled.
To make the frosting, combine Cool Whip with cocoa powder. Fold in crushed Ferrero Rocher. Spread the whipped topping over the cake.
Sprinkle with chocolate chips and some more crushed Ferrero Rocher. Drizzle with chocolate sauce. Refrigerate for about 2 hours. Serve and enjoy!