Chocolate Cake Preparation:

1. Preheat the oven to 330°F (165°C) and grease and line the bottom of two 6-inch (15 cm) round cake pans.
2. In a bowl, mix and sift together the dry ingredients: flour, cocoa powder, baking soda, baking powder, and salt.
3. In another bowl, mix together the buttermilk with the warm coffee (or water).
4. In the bowl of your stand mixer or in a large bowl, whisk together the vegetable oil, sugar, eggs, and vanilla.
5. Alternately add the dry ingredients mixture and the milk and coffee mixture into the oil and sugar mixture, mixing until you have a smooth and liquid cake batter.
6. Pour the batter into the prepared 2 pans and bake in the oven for 45 to 50 minutes until they are well cooked in the center. (Check the doneness by inserting a toothpick into the center of the cakes; it should come out clean.)
7. Let cool slightly in the pans before removing the cakes and allowing them to cool completely top side down on a cake rack.
8. Once completely cooled, slice them into 2 layers using a cake leveler to obtain 4 layers of sponge cake.

Nutella Buttercream Frosting

1. Whisk the egg whites and sugar in a heatproof bowl set over simmering water until the sugar is fully dissolved and the mixture reaches 160°F (70°C).
2. Transfer to a stand mixer and whip on high speed until stiff peaks form and the bowl feels cool.
3. Add the softened butter gradually, mixing until smooth, then beat in the Nutella until creamy and stable.

Nutella Mascarpone Frosting

1. Place the cold mascarpone, cold heavy cream, powdered sugar, Nutella, cocoa powder, and vanilla in a mixing bowl.
2. Whip on low speed, then increase to medium speed until the frosting becomes thick, smooth, and holds soft peaks.
3. Stop mixing as soon as the frosting is firm and pipeable.

Layer Cake Assembly:

1. Prepare a cake turntable stand and place a cake board, then place the first layer of sponge cake.
2. Pipe a border of buttercream around the edge of the cake layer, then fill the center with the mascarpone cream, drizzle some Nutella, and cover with the second chocolate sponge cake.
3. Repeat the last step until the final layer of sponge cake.
4. First, cover the entire cake with a thin layer of buttercream and refrigerate for 20 minutes to lock in the crumbs.
5. Cover again with a thicker layer of buttercream and smooth with a cake smoother, then refrigerate for 1 hour.
6. Prepare the chocolate ganache drip by heating the chocolate and cream in the microwave in 15-second intervals until you have a runny ganache that's not too hot (ideal temperature 95°F or 35°C).
7. Pour the chocolate ganache over the top of the cake and let it drip down the sides.
8. Decorate the top of the layer cake with the remaining buttercream using a 1M piping tip, sprinkle with hazelnuts, and let the cake rest in the refrigerator for 1 hour before serving.
9. Take the cake out of the refrigerator 20 minutes before slicing to soften the cream.