6 eggs, separated
1 1/3 c. confectioner’s sugar (divided use)
1 t. vanilla extract
1 c. plus 2 T. sifted cake flour*
pinch of salt
Chocolate Buttercream Frosting
4 ounces bittersweet chocolate (I used Ghirardelli chocolate chips)
3 sticks unsalted butter, at cool room temp (yes, three sticks)
2 T. cocoa powder
1 ¼ c. confectioner’s sugar, sifted
1 t. vanilla
1 large handful hazelnuts, de-skinned** and chopped (I guess this is optional)
**Since you already have the oven on, throw the hazelnuts on an ungreased cake pan (or cookie sheet, or whatever), and roast them for about 15 minutes, or until the skins look loose, then let cool and rub vigorously in a kitchen towel until most of the skins come off. I may have over-baked mine as the skins turned almost black, but miraculously they didn’t burn and the nut meats actually got nicely golden brown and roasty.
Unless you have removable-bottom cake pans (thanks Bonnie!), I recommend the parchment paper method. In fact, it was even somewhat of a pain to get the layers off the greased-and-floured removable bottoms, so I’d parchment those too. But really this batter is so thick and foamy that you don’t need cake pans at all. Cut out parchment paper a bit bigger than the layer size you want, trace the circle (or rectangle, or whatever) that you want on parchment and lay the parchment on a cookie sheet (the cookbook notes that you should have the side of the parchment with the graphite/ink on it facing DOWN so as not to pollute the batter). I made five layers, each 9.5 inches in diameter, and supposedly the batter is enough for 6 layers at 9 inches, or you could do some math and make crazy sizes/shapes. Once you have all that set up (you might have to borrow some cookie sheets), turn the oven on to 400 F with the racks in the center and top third of the oven.
Make and bake batter:
Beat eggs yolks, 2/3 c. confectioner’s sugar and vanilla at high speed for 3 minutes, until they are pale yellow and thick (this is a job for the stand mixer).
Sift half of the flour/salt over top and gently stir in, repeat with other half of the flour and stir until homogenous in color. Batter will be thick and foamy. Plop approximately equal portions of the batter onto your prepared parchment outlines, and spread the batter to the edge of the circles (push, don’t pull) to make a mostly-even layer, about ¼ inch thick, depending on the size and number of your layers.
Bake each layer for about 5 minutes (mine actually took closer to 7, I think my oven’s a bit cool), or until the top springs back when pressed and the edges are golden brown. Be sure to rotate the layers on the oven racks and turn them once; most ovens have a hotter side which will quickly burn these skinny little guys.
When done, flip the layers onto a rack and peel off the parchment, then flip them back onto the parchment or a rack to cool. The tops are very sticky, so don’t leave them sitting around top-side-down for any period of time. Let the layers cool completely, it won’t take long. Don’t worry if some layers are ugly (one of mine ended up with a hole in it), because they will all be lovely ensconced in chocolate frosting very shortly. Any that are particularly large or non-circular should be trimmed so that they stack nicely. (The cookbook recommends laying the bottom of an appropriately-sized springform pan or plate on top and trimming around...I just eyeballed it. I assure you that taste is not affected.)
Make frosting and frost:
Melt chocolate. Cream butter until smooth (for some reason the cookbook recommends using a hand mixer as opposed to a stand mixer for this—maybe to take advantage of the finer metal blades rather than the stand mixer’s paddle?). Mix cocoa into butter, then confectioner’s sugar, then chocolate, then vanilla. Beat until evenly-colored and frostingey. If your kitchen is 90 degrees, I’d recommend throwing the frosting in the fridge for a few minutes while you drink something cold (or wash the dishes) or you end up trying to spread something that’s the consistency of melted butter. Trust me.
When the frosting is spreadable, start stacking and frosting the layers of your cake. It’s a fancy cake, so use a fancy cake plate (thanks Bonnie!). I alternated the ugly layers with the pretty ones, saving one that was fairly flat and uniform for the top layer. Frost tops of all layers as you stack, then frost top and sides of cake. You don’t have to be too sparing with the frosting, but be modest about it or you’ll end up short for the sides. Sprinkle chopped hazelnuts on the top of the cake and refrigerate until eating time.