Classic American Chocolate Layer Cake

Cake:
1 pound butter, softened, plus extra for cake pans
1 and 1/2 cups cake flour plus extra for cake pans
3 ounces unsweetened baking chocolate (good-quality, preferably Valrhona)
6 and 1/2 ounces bittersweet chocolate (good quality, preferably Valrhona)
2 and 3/4 cups granulated sugar
10 eggs, 2 whole and 8 separated
1 tablespoon vanilla extract
14 ounces sour cream
3/4 cup cocoa powder
1 teaspoon baking soda
3/4 teaspoon baking powder
Pinch salt
4 cups chocolate icing (recipe below)
Preheat the oven to 375 degrees.

Chocolate Icing:
12 ounces bittersweet chocolate, chopped (good-quality, preferably Valrhona)
1 and 1/2 cups heavy cream
8 ounces granulated sugar
3/4 cup egg whites (about 6 large eggs)
12 ounces butter, softened

Cake:
Butter and flour three 9-inch-diameter cake pans. In a bowl set over a pan of barely simmering water or in the microwave, melt the two chocolates together, then set the bowl aside. With an electric mixer, beat the butter and 2 cups sugar until they're light and fluffy, about 5 minutes. Add the 2 whole eggs and 8 egg yolks, vanilla, and sour cream and continue beating until well blended. In another bowl, mix together the cocoa and baking soda, then whisk in 3/4 cup hot water until the mixture is smooth. Add the melted chocolates, then the cocoa mixture, to the batter in the mixer bowl. Sift together the flour, baking powder, and salt and fold into the batter. Whip the egg whites in a large bowl until frothy, then add the remaining 3/4 cup sugar, a little at a time, until the whites reach soft peaks; do not overbeat. Whisk a little of the beaten whites into the batter until well blended, then fold in the remaining whites. Pour the batter into the three prepared pans and bake for about 50 to 60 minutes or until the top is springy and the cake begins to separate from the sides of the pan. Unmold onto a wire rack and cool. Level the surface of each cake; spread a layer of chocolate icing between each layer of cake and frost the outside of the cake with the remaining icing. Decorate with chocolate shavings if desired.

Chocolate Icing:
Place the chocolate in a bowl. Bring the cream to a boil in a saucepan and pour over the chocolate, stirring until the chocolate has melted. Set aside in a cool place to set. In a small saucepan, combine the sugar with 1/2 cup water and cook over high heat until the syrup reaches soft-ball stage (240 degrees on a candy thermometer; a spoonful dropped in ice water should form a ball when rolled between the fingers).

Meanwhile, whisk the whites in the bowl of an electric mixer until soft peaks form. Gradually pour in the hot sugar syrup, and continue beating until the meringue cools to room temperature. Whip the butter in the bowl of an electric mixer with the chocolate-cream mixture until it's light and fluffy. Fold in the meringue, and use immediately.