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

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

Add eggs, milk, vegetable oil, and vanilla extract to the dry ingredients and mix until well combined.

Slowly pour in water and gently mix until the batter is smooth and thin.

Divide batter evenly into the prepared cake pans. Bake for 30-33 minutes or until a toothpick inserted comes out clean.

Allow cakes to cool for 10 minutes in pans, then transfer to wire racks to cool completely.

To make the buttercream, beat butter and Nutella until smooth. Gradually add powdered sugar and mix until combined. Add heavy cream as needed for desired consistency.

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

Prepare ganache by heating Nutella, chocolate chips, and heavy cream until melted and smooth. Pour over the assembled cake.

Decorate with Ferrero Rocher candies and serve.