Preheat the oven to 350°F. Prepare a 9x13 inch baking pan with a light coating of cooking spray. Set aside.
In a large bowl, 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.
Poke holes all over the cake using the end of a wooden spoon.
Place the condensed milk, chocolate chips, and Nutella in a microwave-safe bowl. Microwave 1 minute 30 seconds, (adding 20-second intervals if needed.) Whisk until smooth and combined. Reserve about 1/3 cup of chocolate sauce for drizzling over the frosting.
Pour the chocolate sauce all over the cake. Chill cake in the fridge for an hour until completely cooled.
To make the frosting, combine Cool Whip with cocoa powder. Fold in 5 crushed Ferrero Rochers. Spread the whipped topping over the cake.
Sprinkle with chocolate chips and remaining crushed Ferrero Rocher. Drizzle with chocolate sauce. Refrigerate for about 2 hours. Serve & enjoy!