Preheat oven to 350º.
To make the cake, place the unsweetened chocolate in a microwave safe bowl. Gently microwave, using 30 second increments, stirring frequently, until almost all melted. Remove and let sit, stir until smooth. If needed zap again for a few seconds to make sure all chocolate is melted.
Spray two 9-inch cake pans with Baking Pam with flour.
Whisk together the 2 cups of flour, the baking soda, and salt in a medium bowl. Set aside.
Combine the brown sugar and 8 tablespoons soft butter in the bowl of an electric mixer fit with the paddle attachment.
Beat on low for 3 minutes. Scrape down the sides of the bowl, then beat on high for 2 minutes. Scrape down the sides of the bowl again and beat on high for another 1½ minutes.
Add 4 eggs, one at a time, beating on high for 30 seconds after adding each egg. Scrape down the bowl after each addition, then beat on high for 2 more minutes. Add the melted chocolate and the vanilla. Beat on low for 30 seconds, then scrape down the bowl.
Microwave 1 cup of water until near boiling in a microwave safe Pyrex measuring cup.
With the mixer on low add a third of the flour mixture.
Mix, then add ½ cup sour cream; allow to mix for 30 seconds.
Add another third of the flour and the remaining sour cream and mix for another 30 seconds.
Add the remaining sifted flour and the boiling water and mix for an additional 30 seconds before removing the bowl from the mixer. Use a rubber spatula to finish mixing the batter, until it is smooth and thoroughly combined. You do not want to over-mix.
Pour the cake batter into the prepared pans, dividing it evenly. Smooth the tops with an offset spatula.
Bake until a toothpick inserted in center comes out clean, 35-45 minutes. Remove the cakes from the oven and cool in the pans for 15 minutes at room temperature. Remove cakes to cooling racks and refrigerate until needed.
To make the ganache: Heat the heavy cream, 2 tablespoons butter, and 2 tablespoons sugar in a 2½-quart saucepan over medium-high heat, stirring to dissolve the sugar. Bring the mixture to a boil. Place 8 ounces semisweet chocolate and the instant coffee powder in a heatproof bowl. Pour the boiling cream over the chocolate. Allow to stand for 10 minutes, then stir until smooth. Keep at room temperature until ready to use.
To make the buttercream: Heat 1 inch of water in the bottom half of a double boiler over medium heat. Place 8 ounces semisweet chocolate, 2 ounces unsweetened chocolate in the top half of double boiler. Tightly cover the top with plastic wrap. Allow to heat for 8 to 10 minutes, add vanilla and stir until smooth. Set aside until needed.
Place 1 pound of butter in the bowl of an electric mixer fitted with a paddle. Beat the butter on low for 2 minutes then on medium for 3 minutes. Scrape down the sides of the bowl. Beat on high until light and fluffy, about 4 to 5 minutes. Transfer the butter to a large bowl. Set aside until needed.
Heat 1 inch of water in the bottom half of a double boiler over medium heat. Place 5 egg whites and 1 cup sugar in the top half of the double boiler. Gently whisk the egg whites until they reach a temperature of 120 degrees, about 3 to 5 minutes. Transfer the heated egg whites to the bowl of an electric mixer fitted with the whisk attachment. Beat on high until stiff peaks form, about 4 minutes. Remove from mixer.
Fold the melted chocolate into the butter, using a rubber spatula to thoroughly combine. Fold in the whipped egg whites until thoroughly combined. Set aside.
To assemble the cake: Remove cakes from refrigerator and turn them over so the top side is facing up. Trim off the tops of the cakes, if they are rounded. Slice each cake horizontally into 2 equal layers. Place the top layer of a cake onto the bottom of a closed 9-inch springform pan. Evenly spread 1½ cups of the buttercream over the cake in the springform pan. Place the bottom layer of the first cake onto the buttercream and gently press into place. Pour 1¼ cups of ganache over the cake layer, spreading the ganache evenly to the edges. Refrigerate the remaining ganache. Place the top layer of the second cake on top of the ganache and press into place. Spread 1½ cups buttercream evenly over this layer. Place the remaining bottom cake layer, cut side down, onto the buttercream and gently press into place. Cover the entire cake and pan with plastic wrap and freeze for 1 hour.
To decorate the cake: Fill a pastry bag fitted with a large star tip with 1½ cups of buttercream. Remove the cake from the freezer. Run a metal spatula or table knife around the edges next to the pan to release the cake from the springform pan. Transfer the cake to a serving plate. Using a cake spatula, evenly spread the remaining buttercream over the top and sides of the cake. Refrigerate the cake for 1 hour.
