Combine espresso, hot water, semisweet chocolate, and bitter chocolate in double boiler filled with hot water and cover. Without applying additional heat, melt the chocolate.
Butter two layer-cake pans, lay a sheet of waxed paper in each, and dust the pans with flour. (The pans should be able to hold 4 cups each.)
Beat the egg yolks and then gradually add 2/3 cup of sugar until the yolks are thick and lemon colored.
Beat butter into the chocolate mixture. Add the chocolate mixture to the egg yolks and stir.
Beat the egg whites until foamy. Add the cream of tartar, and then gradually add up to 2 Tbsp sugar.
Add the sifted corn starch gradually to the chocolate mixture, alternating with the egg whites.
The corn starch and egg whites should be folded in to the chocolate mixture.
Pour half the batter into each cake pan.
Bake on the lower middle level of a preheated 350 degree F oven until set on the sides.
Cool the cake in the pan, and then turn out onto a serving dish.
Combine the icing ingredients and make the icing as per Step 1. Ice the cake as you would a normal layer cake.
