Preheat oven to 300°F. Prepare three 8-inch cake pans with parchment paper and grease the sides.

Add flour, sugar, cocoa powder, baking soda, and salt to a large bowl and whisk together.

Add eggs, milk, vegetable oil, and vanilla extract and mix until well combined.

Add water and gently mix until the batter is smooth and thin.

Divide batter evenly into the pans and bake for 30-33 minutes until a toothpick comes out clean.

Cool cakes for 10 minutes, then transfer to racks to cool completely.

Prepare the buttercream by beating butter and Nutella until smooth.

Gradually add powdered sugar and mix until combined.

Add heavy cream a tablespoon at a time until the desired consistency is reached.

Level the cakes, then spread buttercream between layers and on the outside.

Make ganache by heating Nutella, chocolate chips, and heavy cream until smooth. Pour over the cake and smooth evenly.

Pipe remaining frosting around the top edge, decorate with Ferrero Rocher candies.

Refrigerate if not serving immediately. Serve at room temperature.