Since the flavor of these layers depends entirely on the chocolate, please use the best brand you can afford- I like Callebaut, but you can also use Lindt, Tobler, Valrhona or Ghiardelli.
Using a standard loaf pan, line it with plastic wrap so that the plastic extends beyond the sides of the pan by at least 3 inches on each side.
For the dark chocolate layer: melt the chocolate, espresso, and rum in the microwave on 50% power for about 2 minutes.
Stir until smooth.
Whisk in the yolk and mix well.
Set aside.
Whip the whites and cream of tartar until soft peaks form.
Sprinkle in the sugar, beating on high until stiff, but not dry.
Fold a quarter of the whites into the chocolate to lighten it, then scrape the remaining whites on top of the chocolate and set aside.
Beat the cream until it holds it shape (not stiff) then scrape on top of the whites.
Fold everything together gently until no streaks of white appear.
Turn the mousse into your prepared mold, spreading until it is an even layer.
Place in the freezer.
Make the milk chocolate layer: melt the chocolate and coffee in the microwave on 30% (low) power for about 2 minutes.
Stir until smooth and completely melted.
(Milk chocolate burns very easily. Do not attempt to melt it at a higher temperature.) Set aside to cool slightly.
Whip the cream until soft peaks form- not too stiff.
Fold very carefully into the milk chocolate- it will seem very soft.
Scrape on top of the dark chocolate layer, smoothing to an even layer.
Put in the freezer.
Make the white chocolate layer: place the chocolate and 2 tbl.
water in a heatproof bowl.
Microwave on low (30% power) for about 2 minutes.
Stir until very smooth and completely melted.
Let cool slightly.
Whip the cream until soft peaks form- not too stiff.
Fold very carefully into the chocolate- it, too, will seem very soft.
Scrape on top of the milk chocolate layer, smoothing to an even layer.
Now- gently, gently tap the pan a couple of times on the counter to allow all the layers to settle against one another.
Carefully wrap the excess plastic up over the mousse.
Put the entire thing in the freezer for at least 4 hours, preferably overnight.
It also freezes well for a month prior to serving.
To serve: Take the pan from the freezer, and set in a pan of hot water for 10 seconds.
Carefully unwrap the plastic from the top, then unmold onto a cutting board, pulling on the plastic wrap to help release it.
Peel the plastic from the mousse and discard.
Slice the mousse into 8 slices (this is done more easily with a knife you run under hot water then wipe dry quickly and cut while the knife is still warm) and carefully arrange on individual plates.
Serve immediately.
Serving suggestions: raspberry sauce and a few berries and a sprig of mint, or be really decadent and serve it with a drizzle of hot fudge sauce and a sprig of mint or both.
