Special equipment: Two 9 by 2-inch round cake pans, buttered, each coated with 1 tablespoon sugar
Preheat oven to 350 degrees F.
For the cake: In a large mixing bowl of a standing mixer, fitted with whisk attachment, combine eggs and 1 cup of sugar. Beat on medium-high speed until the mixture is pale yellow, thick, and has tripled in volume, about 8 minutes. In a separate, large mixing bowl, sift together the cocoa, flour, and baking powder. Fold cocoa mixture into the egg mixture to combine completely, until mixture is smooth. Divide cake batter evenly between 2 cake pans. Bake for 25 minutes, or until cake springs back when touched. Cool cakes for 15 minutes. Loosen cakes from pans with an offset spatula, and unmold onto wire rack. Cool to room temperature.
For the Pudding: Combine 1/2 cup of the cream with the cornstarch in a small bowl and stir to make a paste. Combine paste with remaining 3 1/2 cups cream, sugar, chocolate chips, and vanilla in a large nonstick saucepan. Using a wire whisk, stir the mixture until it is well blended. Over low heat, whisk cream mixture until chocolate melts thoroughly. Cook the mixture, stirring frequently, until it becomes very thick, about 25 minutes. Pour pudding into a large bowl. Cover with plastic wrap, covering surface of pudding with the plastic to prevent a skin from forming. Let cool to room temperature.
To Assemble the Cake:* Line a baking sheet with parchment 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. Chill in the refrigerator for 2 hours.
For the Ganache: In a medium-sized nonstick saucepan over medium heat, heat cream just under boiling point. Remove from heat and add chopped chocolate. With a whisk, stir until chocolate is completely melted and mixture is smooth. 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. Pour ganache over the top of chilled cake, allowing 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:
In a medium-sized saucepan, add 3-inches of water. Boil water and reduce to a simmer. Place semisweet chips in stainless-steel bowl and place bowl over simmering water. Stir chocolate until melted to smooth consistency. Line a baking sheet with parchment and spread melted chocolate evenly across sheet. Chill in refrigerator until set, about 1 hour. Break the chocolate into large, shard-like pieces. To garnish on cake, place chocolate shards on top of the cake vertically at various angles. Sprinkle with confectioner's sugar.
*If necessary, shave off any uneven pieces of cake with a serrated knife so that it is smooth and even on all sides.