Preheat the oven to 350ºF.
FOR THE CAKE:
Put the eggs and 1 cup of the sugar in a large mixing bowl, and with an electric mixer fitted with a wire whisk beat on medium-high speed until the mixture is pale yellow, thick, and has tripled in volume, about 8 minutes, using the mixer.
Sift the cocoa, flour, and baking powder together in another large mixing bowl. Add the egg mixture and fold to mix thoroughly so the mixture is smooth.
Grease two 9 x 2-inch round cake pans with the butter. Sprinkle each with a tablespoon of the remaining sugar. Pour the cake batter evenly into the pans and bake until the cakes spring back when touched, about 25 minutes.
Let cool for about 2 minutes. Using a thin knife, loosen the edges of the cakes, then flip onto a wire rack. Let cool completely.
FOR THE PUDDING:
Combine 1/2 cup of the cream with the cornstarch in a small bowl and stir to make a paste. Combine the paste with the remaining 3 1/2 cups cream, the sugar, the chocolate chips, and the vanilla in a large nonstick saucepan. Using a wire whisk, stir the mixture until it is well blended. Put the saucepan over medium-low heat and whisk constantly until the chocolate melts thoroughly. Cook the mixture, stirring often, until it becomes very thick, like pudding, about 25 minutes. Pour the mixture into a bowl. Cover with plastic wrap, pressing the wrap down on the surface of the pudding to keep a skin from forming. Let cool to room temperature.
TO ASSEMBLE THE CAKE:
Line a baking pan with parchment paper or waxed paper and place a wire rack over it. Using a serrated knife, cut each cake in half horizontally. Brush the tops of three layers each with 1/4 cup of the Grand Marnier. Place the bottom layer on a 9-inch round of cardboard and set it on the wire rack. Spread 1 1/2 cups of the pudding evenly on top of the layer. Top with a second layer of cake. Spread 1 1/2 cups of pudding evenly over it. Repeat the same process with the third layer. Top with the fourth layer. If necessary, shave off any uneven pieces of cake with a serrated knife so that it is smooth and even on all sides. Chill for 2 hours.
TO MAKE THE GANACHE:
Combine the cream and chopped chocolate in a medium-size nonstick saucepan over medium heat. Stir until the chocolate is completely melted and the mixture is smooth. Remove from the heat and stir to cool, lifting the mixture out of the pot several times with a rubber spatula or wooden spoon until it cools slightly. It should be glossy and slightly thick. This is the tempering process.
Spoon the mixture onto the top of the chilled cake, allowing the overflow to drip down the sides. Cool slightly. Carefully remove the cake from the wire rack. Chill for at least 6 hours.
FOR THE GARNISH:
Melt the semisweet chocolate chips in a stainless steel bowl set over simmering water. Stir until the chocolate has melted and is smooth.
Line a baking sheet with parchment or waxed paper. Pour the chocolate onto the baking sheet and spread evenly. Let cool, then chill until it sets. Break the chocolate into pieces, like brittle. Mound the pieces on top of the cake, sticking them in at various angles. Sprinkle with confectioners’ sugar. Slice and serve.
Yield: One 9-inch layer cake; 10 servings