Preheat oven to 350 degrees. Grease 3 8-inch round cake pans and line with parchment paper.
Place the 4 oz. German sweet chocolate, broken into pieces, in a medium bowl and pour the 1/2 cup boiling water over top. Add 1/2 teaspoon espresso powder, if using, and let stand for 1-2 minutes. Whisk together until the chocolate is melted and smooth. Set aside to cool to room temperature.
In a large bowl whisk together 2 cups all-purpose flour, 1/2 teaspoon salt, and 1 teaspoon baking soda. In the bowl of a stand mixer, cream 1 cup butter and 2 cups sugar until light and fluffy. Add 4 egg yolks, one at a time, beating well after each addition.
Stir the cooled chocolate mixture and 1 teaspoon vanilla into the butter/sugar mixture. Add the flour mixture, alternately with the 1 cup buttermilk, starting and ending with the flour mixture.
In a separate bowl, mix the egg whites with an electric mixer on high speed until stiff peaks form. Gently fold the egg whites into the cake batter.
Divide the batter evenly between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool on wire racks for 10-15 minutes before turning out to cool completely.
Prepare the German chocolate frosting. Place 1/3 of the frosting on top of 1 cake layer and spread evenly to the edges. Top with a second layer and repeat the frosting. Top with the final layer and spread the remaining frosting evenly over the top of the cake. You will not frost the sides of the cake.
Garnish with pecan halves or toasted coconut, if desired. Serve at room temperature.