Separate the Large Egg (4) whites and yolks.
Heat a saucepan one-third full with hot water, and in a bowl set on top, melt together the Bittersweet Chocolate (6 ounce), Unsalted Butter (3/4 cup) and the Dark Roast Coffee (1/4 cup) stirring over the barely simmering water, until smooth. Remove from heat.
Fill a large bowl with ice water and set aside.
In a bowl large enough to nest securely on the saucepan of simmering water, whisk the yolks with Granulated Sugar (1/2 cup), Dark Rum (2 tablespoon), and Water (1 tablespoon) for about 3 minutes until the mixture is thick, like runny mayonnaise.
Remove from heat and place the bowl of whipped egg yolks within the bowl of ice water and beat until cool and thick. Then fold the chocolate mixture into the egg yolks.
In a separate bowl, beat the egg whites with Salt (1 pinch) until frothy. Continue to beat until they start to hold their shape. Add 1 tablespoon of sugar and continue to beat until thick and shiny, but not completely stiff. Then add the Vanilla Extract (1 teaspoon).
Fold one-third of the beaten egg whites into the chocolate mixture, then fold in the remainder of the whites just until incorporated, but don’t overdo it or the mousse will lose volume.
Transfer the mousse to a serving bowl or divide into serving dishes, and refrigerate for at least four hours, until firm.
Top with Dark Chocolate (to taste) if desired.