Adjust rack in lower third of oven. Preheat oven to 350 degrees. Grease and flour two 8-inch round cake pans, and insert parchment paper or waxed paper to line the bottoms. Sift the flour, baking soda, and salt onto a sheet of waxed paper; set aside. Place the cocoa in a 1-quart mixing bowl. Add the 1/2 cup lukewarm water, and whisk to combine; set aside to cool. Pour the buttermilk, the 1/2 cup water, and the vanilla into a liquid cup measure. Crack the eggs into a small bowl, and whisk together to combine the yolks and whites.
Place the butter in the bowl of a heavy-duty mixer, preferably fitted with a flat beater. Cream the butter on medium speed until the butter is smooth and lighter in color, about 30 to 45 seconds. Reduce the speed to low, add the sugars in a steady stream. When all the sugar is added, stop the machine, and scrape the mixture clinging to the side of the bowl into the center. The mixture will appear sandy. Increase the speed to medium again, and cream until the mixture is light in color, is fluffy in texture, and appears as one mass, about 5 to 7 minutes. With the mixer still on medium speed, pour the eggs slowly at first. Continue to cream, scraping the sides of the bowl at least once, until the mixture appears fluffy and velvety. Stop the machine and spoon in the cooled cocoa mixture, resume at medium speed and mix just until incorporated.
Using a rubber spatula, stir in one-fourth of the flour mixture. Then one-third of the buttermilk mixture, stirring to blend together. Repeat this procedure, alternating dry and liquid. With each addition, scrape the sides of the bowl, and continue mixing until smooth, never adding liquid if any flour is visible.
Pour the batter into the pans and spread it level. Bake for 25 minutes, or until the baked surface springs back slightly when touched lightly in the center and the sides contract from the pan.
Place the cake pans on a rack to cool for 5 to 10 minutes. With mitts, tilt and rotate pans gently tapping them on the counter to see if the cake releases from sides.
Put the chocolate in a medium bowl. In a small saucepan, heat the heavy cream just to the boil. Remove from heat. Pour over chocolate and whisk until chocolate melts and mixture is smooth and shiny.
ASSEMBLING THE DESSERT:
Split each layer of cake in half horizontally and place one layer, cut side up, on a cardboard round. Spread with one-third of the ganache filling. Center a second layer on top of the first, and spread it with one-third of the ganache filling. Place another layer, cut side up, on top, and spread with remaining filling and turn he last layer upside down, and center it over the filling.
FROSTING AND CHOCOLATE GLAZE
Frost the cake with the whipped cream. Place the cake on a wire rack, then set it on a sheet pan that will fit the dimensions of your freezer. Place in the freezer for 40 minutes only, (this is just enough time to chill the whipped cream frosting so it's firm to the touch, not soft.) While the cake is in the freezer, prepare the dark chocolate glaze. Put the chocolate in a medium bowl. In a small saucepan, heat the heavy cream just to the boil. Remove from heat. Pour over chocolate and whisk until chocolate melts and mixture is smooth and shiny. Set aside to cool to body temperature.
Set the cake on its wire rack over a baking pan with sides (like a jelly roll pan) on top of a turntable (or lazy susan.) Pour almost all of the chocolate glaze over the center of the cake. Using a long, flexible metal icing spatula, use just a few strokes to spread the glaze over the top of the cake so the glaze runs down over the sides. Rotate the turntable as you spread. Use the spatula to scoop up excess glaze to touch it to any bare spots on the sides of the cake to cover them. Place the cake on a serving plate. Refrigerate until 30 to 60 minutes before serving. Using a serrated knife, dipped into hot water after each slice, cut dessert into wedges.