Grated chocolate, cookie crumbs or chopped nuts for coating the sides of cake, if desired
Lightly coat the insides of 3 9 by 1 1/2-inch cake pans with melted butter. Line each pan with parchment paper, then lightly coat the parchment paper with more melted butter. Set aside.
Preheat the oven to 325 degrees Fahrenheit.
Heat 1-inch of water in the bottom half of a double boiler over medium heat. Place remaining butter and 8 ounces semisweet chocolate in the top half of the double boiler. Tightly cover the top with film wrap. Allow to heat for 10 to 12 minutes. remove from the heat, stir until smooth, and hold at room temperature.
Place the egg yolks and 3/4 cup sugar in the bowl of an electric mixer fitted with a paddle. Beat on high until slightly thickened and lemon-colored, about 4 minutes. Scrape down the sides of the bowl and beat on high for an additional 2 minutes.
While the egg yolks are beating, whisk 4 egg whites in a large stainless steel bowl until stiff, but not dry, about 3 to 4 minutes.
Using a rubber spatula, fold the melted chocolate mixture into the beaten egg yolk mixture. Add a quarter of the beaten egg whites and stir to incorporate, then gently fold in the remaining egg whites.
Divide the batter between the prepared pans, spreading evenly, and bake in the preheated oven until a toothpick inserted in the center comes out clean, about 25 to 30 minutes. Remove the cakes from the oven and allow to cool in the pans for 15 minutes. (During baking, the surface of the cakes will form a crust; this crust will normally collapse when the cakes are removed from the oven.) Invert the cakes onto cake circles, remove the parchment paper, and refrigerate for 30 minutes.
To prepare the dark chocolate cream, heat 1-inch of water in the bottom half of a double boiler over medium heat. Place the semisweet chocolate in the top half of the double boiler. Tightly cover the top with film wrap and allow to heat for 8 to 10 minutes. Remove from the heat and stir until smooth. Transfer the melted chocolate to a stainless steel bowl and set aside until needed. Place heavy cream and sugar in the well-chilled bowl of an electric mixer fitted with a well-chilled balloon whip. Whisk on high until stiff peaks form, about 1 1/2 minutes. Remove the bowl from the mixer. Use a hand whisk to combine 1/4 of the whipped cream into the melted chocolate until smooth and completely incorporated. Add the combined whipped cream and chocolate to the remaining whipped cream and use a rubber spatula to fold together.
Heat the heavy cream and the butter in a 3-quart saucepan over medium high heat. Bring to a boil. Place the semisweet chocolate in a 3-quart stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Chill 1 cup of ganache for at least 1 hour. Remaining ganache should be at room temperature
To assemble cake:
Arrange one of the cake layers in a 9-inch springform pan, evenly spread half the filling over the cake and top with the second layer. Add remaining filling and top with the remaining cake layer. Chill 2 hours. Transfer chilled cup of ganache to pastry bag fitted with a star tip. Remove cake from springform pan and coat sides and top with room temperature ganache. Using chilled ganache, decoratively pipe stars on top of cake. If desired, coat sides of cake with grated chocolate, cookie crumbs of chopped nuts.