For the Cake
Before anything else, arrange your oven rack in the middle and start preheating your oven to 350 degrees Fahrenheit.
After that, take three cake pans of the same sizes, 8 or 9 inches, and line them with parchment paper. Then, spray your cake pans with a non-stick cooking spray.
Afterward, combine all the dry ingredients; cake mix, flour, sugar, cocoa powder, salt, and whisk until everything combines well.
In a separate large bowl, incorporate all the wet ingredients; eggs, vanilla, oil, water, and sour cream.
Then, use your electric mixer, combine the wet ingredients with your dry ingredients. Start beating at low speed and gradually increasing to medium-high. Beat all the ingredients until the mixture combines well.
Scrape the bowl's sides, make sure that all the ingredients incorporate properly, and then beat for another minute.
Afterward, pour the cake batter proportionally into the prepared cake pans. Then, bake for about 20 to 25 minutes or until cooked(check by inserting a toothpick)
Next, please take out the cake from the oven and let it rest for 15 to 20 minutes. If the cake has rested, remove it from the cooking pan. Then, peel off the parchment paper and allow it to cool in the wire rack to cool it completely.
For the Nutella Frosting
First and foremost, using a double boiler, melt the chocolates (semi-sweet and white chocolate). Set it aside.
Then, beat the butter using an electric mixer in a large mixing bowl. Do this until you can achieve a smooth and creamy consistency. After that, put in the softened cream cheese and beat for another 1 minute until the mixture combines thoroughly.
Incorporate the Nutella with the mixture and beat for another minute. Scrape the bowl's side to combine the mixture properly.
Add the melted chocolate mixture to the frosting mixture and beat it until everything combines well.
After that, cover the bowl with plastic wrap. Then, please place it in a cool place until it is ready to use.
For the Nutella Filling
In a medium bowl, combine Nutella and milk. Whisk until everything is completely smooth.
Cover the Nutella filling mixture with a cling wrap and set it in a cool place until ready for usage.
For Assembling the Cake
Cut your cakes horizontally using a long serrated knife. Then, cut each cake into two even and thin layers. (You should have about six thin layers of cakes.)
Afterward, place your cake on a cake board. Then, make sure to add a bit of frosting to the cake board. Doing this ensures that the first layer of cake sticks. Next, add the Nutella filling on top of the first layer of the cake. Finally, make sure to spread it evenly using an offset spatula. Afterward, repeat the process until all of it stacks with one another. If you feel that your cake is unstable, you can secure it by putting dowels in the cake.
After that, crumb coats your cake using a small amount of frosting and an offset spatula. Then, chill it for 10 minutes or until the frosting is firm.
Then, cover your cake with Nutella frosting and begin designing your cake. After that, you can pipe the top of the cake and add sprinkles, chocolates, or anything you like.
Finally, you can serve the cake as soon as possible or cover and refrigerate it before serving.