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. Place the granulated sugar with the egg whites in a heatproof bowl and heat the mixture over a double boiler on a pot containing a little heated water over medium heat.
2. Whisk constantly using a hand whisk while monitoring the temperature with a cooking thermometer, which should read at least 122°F (50°C). (The sugar should be completely dissolved.)
3. Transfer the mixture to the bowl of your stand mixer fitted with the paddle attachment and beat on high speed for about 15 minutes until you get a stiff and cooled meringue.
4. Reduce the mixer speed and gradually incorporate the softened butter pieces, then increase to high speed again for about 5 - 10 minutes until you get a thick cream (the mixture will have a grainy consistency and then become smoother).
5. Add the vanilla and Nutella and mix again for 2 minutes, then switch to the flat beater and mix at low speed for 2-5 minutes to loosen and smooth the buttercream.

Nutella Mascarpone Frosting:

1. In a large bowl, place the cold mascarpone, cold heavy cream, confectioners' sugar, sifted cocoa powder, Nutella, and vanilla.
2. Whisk at medium-low speed with an electric mixer for 1-2 minutes until you get a thick and creamy frosting.

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.