Preheat the oven to 350 degrees F. For the cake: Put the eggs and 1 cup of 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 by 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.
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 wore rack. Spread 3/4 cup of the frosting evenly on top of the layer. Top with a second layer of cake. Spread another 3/4 cup 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. If should be glossy and slightly thick. Spoon the mixture onto the top of the 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
In a saucepan, over high heat, add the sugar and water. Bring the mixture to a boil and cook until the mixture reaches 240 degrees F, soft ball stage. Using a mixer, fitted with paddle, beat the egg yolks. With the machine running, slowly add the syrup. Beat until the mixture is thick and cool, about 5 minutes. In a mixing bowl, beat the butter well. Add the butter, a tablespoon at a time and beat well. Add the caramel sauce and beat well. Remove the frosting from the mixer.
Yield: 2 1/4 cups