Preheat your oven to 350°F Place a rack in the lower third of the oven and set a baking stone on it. (The stone helps the cake bake evenly and prevents humped tops. If you don't have a stone, the cake will survive don't worry. Just be prepared to trim any uneven tops and make sure the cake gets an adequate amount of time in the oven to fully cook.).
You'll need two 9" or two 8"cake pans for this cake.
Spray the pans with non-stick spray oil and then line them with parchment (sides and bottom--this is a sticky cake) then spray the parchment with even more non-stick spray.
In a medium sauce pan, combine the sugar, dutch processed cocoa, salt and baking soda.
Stir with a whisk until combined and then slowly add the boiling water while whisking.
This will bubble up a bit while the baking soda reacts with any remaining acidity in the dutch processed cocoa.
Once throughly mixed, place the sauce pan over medium-high heat and bring back to a boil.
Then remove from heat and allow to stand for a minimum of 10 minutes.
This step makes the dutch processed cocoa even more alkaline, so our cake will be nearly black.
Measure out the flour, buttermilk (no weird substitutions, folks. We need the acidity), and separate the eggs.
Add the oil and vanilla to the bowl of your stand mixer with the paddle attachment (this can also be done by hand with a wooden spoon).
When the chocolate mixture is ready, stir in the instant espresso powder. (This won't make the cake taste like coffee, rather it enhances the chocolate flavor of the cake. Dutch processed cocoa has great color but it isn't quite as flavorful as it's light-colored acidic cousin, natural cocoa.)
Add the chocolate mixture to your stand mixer and mix on low speed for about 20 seconds.
Add the flour and continue to mix on low until combined.
With a minimum amount of beating, add the egg yolks, eggs, vanilla and butter milk. The batter will be very thin.
Divide the batter between your two pans and place in the oven on your baking stone.
Bake for 25-30 minutes (for 9" rounds. Add 5-7 more minutes for 8" rounds) until the center of the cake springs back when you touch it.
Allow the cake to cool for 10 minutes on a wire rack.
Run a thin knife around the edge of the cake to loosen and invert onto a flat dish lined with parchment.
Allow to cool completely then wrap the rounds individually in plastic and place in the refrigerator to chill before assembling.
In a heat safe bowl, beat the eggs and sugar together until the mixture is uniform and light in color.
Place over a double boiler and whisk until slightly thickened.
A ribbon of the egg sugar mixture should flow back into the bowl when the whisk is lifted and the sugar should just be beginning to dissolve.
Add the Frangelico (or water) and then continue to whisk over the simmering water until the mixture hits roughly 160°F and coats the back of a spoon.
Remove from heat and add the finely chopped chocolate.
Stir until your arm cramps or the mixture becomes cool to the touch, whichever comes first (roughly 10 minutes).
Set the chocolate aside and beat the heavy cream to soft peaks. Fold the cream into the chocolate, then cover and chill for 4 hours.
Assemble your cake by placing one round onto your serving platter.
Reserving 3/4 cup for garnish (keep chilled), spread the mousse to form an even layer on top of the cake.
Top with the second cake round, touching up the sides with an offset spatula if necessary.
Place the cake in the refrigerator to keep cool and prepare the ganache.
Measure out the chocolate into a bowl.
Bring the butter and heavy cream to a low boil over medium-high heat and pour over the chocolate.
Give the chocolate a shake to submerge and allow to stand for 5 minutes without stirring.
When the time has passed, then you may gently stir the ganache until smooth.
Pour the ganache onto the cake, smoothing over the top with an offset spatula.
Return the cake to the fridge and allow the ganache to cool.
When cool, you can put the finishing touches on the cake.
Take the reserved chocolate mouse and fill a piping bag, fitted with a large star tip.
Drop 6-8 splotches of mousse around the perimeter of the cake and top with chocolate covered coffee beans or malted milk balls.
Keep the cake chilled until ready to serve.
Then allow to stand for 10 minutes before cutting and serving.
Store the cake in the refrigerator, covered to keep it from absorbing odors, for up to 5 days.