It may look hard, but it's much easier than it looks.
For easy assembly I made the layers the night before and left them in the freezer overnight with plastic wrap around them, they didn't get freezer burned or ruined and it took about 15 minutes to finalize the whole cake with frosting.
LIGHTER THAN AIR CHOCOLATE CAKE
For cake layers:
12 oz fine-quality bittersweet chocolate (not unsweetened), chopped
6 tablespoons water
12 large eggs, separated, at room temperature
1 1/3 cup sugar
½ teaspoon salt
2 tablespoons Dutch-processed unsweetened cocoa powder
2 cups heavy cream
6 tablespoons confectioners sugar, sifted
4 tablespoons Grand Marnier*
* I substituted 4 tablespoons cocoa and 1 teaspoon vanilla
To Make cake layers: Preheat oven to 350°F. Grease four 9-inch circular cake pans and line bottoms of circles with a piece of parchment paper.
Melt chocolate with water in a small heavy saucepan over very low heat, stirring. Cool to lukewarm.
In your stand mixer, beat yolks, 2/3 cup sugar, and salt until thick and pale for about 5 minutes. Stir in melted chocolate until combined. In a separate HUGE bowl, beat 12 egg whites until they hold stiff peaks (clean your beaters). I use hand held mixer with a very large stainless steel bowl for this-
Add remaining sugar and beat until whites just hold stiff peaks. Fold one third of whites into melted-chocolate mixture, then fold in remaining whites carefully.
Spread batter evenly over four baking pans. Bake until the cake has puffed and the top is dry to the touch, maybe 16 minutes. I rotated my cake pans once to ensure they cooked evenly. Once the cake is done, cool the cake pans on cooling racks for about 10 minutes.
If you wish, sift cocoa powder over top of cake layers. Then place a piece of waxed paper over the top of the cake pans. Place a cookie sheet over paper and invert cake onto it, gently peeling off wax paper lining. For best results, place all of the layers in the freezer for at least 2 hours or until they are firm enough to be carefully lifted without breaking.
In order to make the filling: In a clean large bowl, beat cream with powdered sugar and Grand Marnier (or cocoa powder) until it just holds stiff peaks.
To assemble the cake:
Bring first cake layer out of the freezer (leave the others in the freezer until you are ready for them) and arrange on your cake plate, cocoa side down. Spread 1/4 of filling evenly over the cake layer. Bring the next cake layer out of the freezer, placing it gently over the filling, again cocoa side down. Repeat until the cake is assembled!
Keep cake in the refrigerator until serving time. The cake layers should thaw after about 2-3 hours of being in the refrigerator and the cake will be ready to serve.