Ganache: Heat the heavy cream in a 2-quart heavy-bottomed saucepan until bubbles begin to form around the edge of the pan. Place the chopped chocolate in a medium-sized mixing bowl. Gradually pour about half of the hot cream over the chocolate. Let stand for 30 seconds to melt chocolate. Slowly whisk it until smooth and homogenous. Add the remaining cream gradually--the shock of the hot cream can cause the fat in the chocolate to separate or break, if added too quickly--and mix until all of it is incorporated and the ganache is smooth and homogenous. It should be thick, shiny and smooth. Add the Grand Marnier and mix to fully incorporate. Pour the ganache onto a parchment paper-lined baking sheet to cool. When the ganache has cooled to the consistency of toothpaste but is still easily spreadable, it is ready to be used. If the ganache has set too much, soften it by heating a small amount in a saucepan placed over medium heat until melted. Then quickly, but gently, stir the melted ganache into the harder, cooler ganache. If its still too hard, repeat the procedure until the desired consistency is reached.
Cake layer: Preheat the oven to 325 degrees F. In a medium bowl with an electric mixer, combine corn syrup, egg yolks, butter and almond flour. Beat at medium high speed until light and fluffy. Add dark rum (or substitute vanilla and orange zest).
In a large bowl with an electric mixer beat egg whites until foamy. Add sugar 1 tablespoon at a time. Increase mixer speed to medium high and whip to stiff but not dry peaks, about 7 minutes.
Fold the meringue into the butter mixture all at once. Using a rubber spatula fold up from the bottom of the bowl, careful not to deflate the meringue. When the two mixtures are just incorporated, add the warm chocolate. Fold again, carefully. Use a whisk, if the meringue does not fold in well.
Pour the batter into a parchment paper-lined baking sheet and spread it evenly to the edges with a rubber spatula. Careful not to smash the batter or it will lose its air and the cake will be flat and chewy. Bake until the cake springs back when touched, or a tester inserted in the center comes out clean, about 35 minutes. Cool cake in the pan on a wire rack. The cake will collapse slightly as it cools.
Using an offset spatula spread the ganache onto the cake layer. Spread the ganache all the way to the edges. If the cake is uneven, use the ganache to make it flat and even as possible. Place the cake in the refrigerator and allow the ganache to harden, about 20 minutes. Then use the offset spatula to make the edges as clean and straight as possible. Place the frosted cake in the refrigerator for about 1 hour to set.
When ready to serve, remove cake from the refrigerator and allow to come to room temperature for at least 30 minutes before serving. Heat a chefs knife in hot water; wipe off excess water and cut cake into the serving size preferred.
Decorations: Spread about a 1/16-inch thick layer of the tempered chocolate over a transfer sheet (see note below). Use a chefs knife to cut the chocolate, but not the transfer sheet, to match the size of the cake pieces. Set aside and let the chocolate set about 15 minutes to an hour, depending on the air temperature. When the chocolate has set, lift each pre-cut piece from the transfer sheet and place it, design side up, on top of the individual cake serving