To make the marble cake:
Preheat oven to 350degreesF. Prepare two 8-inch cake pans with parchment rounds and grease. Set aside. Sift together the cake flour, baking powder, baking soda and salt, set aside.
Cream together the butter and sugar until fluffy, about 3 minutes. Add the eggs, one at a time and mix after each, until well combined. Add the sour cream and vanilla.
Add the dry mixture in 3 batches, alternating with the milk. Split the batter in half and sift the cocoa powder over one half. Stir gently to combine.
Place some of the white batter in the bottoms of the two 8-inch cake pans. Add the chocolate batter in blobs. Very carefully stir the white batter into the chocolate, to achieve a marble pattern. Bake for about 25-30 minutes or until a cake tester comes out clean. Once baked cool on a rack.
While the cake is cooling, prepare the buttercream:
In the bowl of a stand mixer combine the egg whites and sugar. It will be very thick and grainy. Put the bowl over a double boiler and stir it with a rubber spatula until the sugar is completely melted. Brush the sides down with the spatula to make sure all the sugar is melted.
Feel the egg mixture between your fingers to check for graininess. Once it is completely smooth, put the bowl on the mixer and beat with the whip attachment on medium high speed. Whip until it is light, fluffy, glossy and the bowl feels just about room temperature. If the egg whites are not cooled off sufficiently it will melt the butter when you add it.
Once the egg whites are whipped and cooled, add the butter, 2 tablespoons at a time on medium speed. WARNING: After you have added about half of the butter, it may look curdled and runny, this is normal and you should continue adding the rest of the butter.
Once you have finished adding the butter and it has mixed on medium speed for about a minute the buttercream will be creamy and glossy looking again. Add the salt, and vanilla. Divide the buttercream in two bowls, 1/3 in one bowl and 2/3 in the other. Add the nutella to the bowl with 2/3 of the buttercream. Add the cooled, melted chocolate to the other.
Unmold the cooled cakes and cut each later in half. Spread about 3/4 cup of the chocolate buttercream on the first layer. Repeat with the next two layers of cake and buttercream. Top with the final layer of cake.
Top the cake with 1 cup of the nutella buttercream. Spread it smooth with a metal spatula. Put more nutella buttercream on the sides of the cake, using enough to cover all the crumbs. Once the whole cake is covered, smooth the icing with the spatula, until it is nice and flat.
To make the ganache spider web:
In a double boiler melt the chocolate and the butter together, gently whisking until smooth.
Use a parchment bag to pipe a thin spiral around the cake. Use a toothpick or skewer to draw lines from the center of the spiral out to the edge to create the web. Use the chocolate ganache to draw a line down the side of the cake and pipe a smaller spiders body there. Repeat with as many spiders as you like.
Cover the rest of the chocolate ganache and allow it to cool to room temperature. Once the chocolate ganache is set up - this may take an hour or two - place it in a pastry bag fit with a round tip. Pipe a large ball of the ganache to create the spider's body.
Chocolate spider legs:
Fill another paper pastry bag about 1/3 full with the melted chocolate and pipe out the chocolate in the shape of the number 7 onto a silpat, waxpaper or parchment paper. Make sure you do a size that will fit the size of the spider body you intend to have. Pipe several extra in case you have breakage. Place the sheet into the freezer to set the chocolate.
Remove the spider legs from the freezer and using a small spatula, lift up the legs and place them on the spider, 4 on each side. Work quickly so the chocolate doesn't warm up too much. You may have to return the legs to the freezer if you are doing several spiders.