1. In a medium to large bowl set over a saucepan of barely simmering water, melt together the chocolate, butter and coffee, stirring until smooth. Remove the bowl from the heat and let sit until tepid.
2. Fill a large bowl with ice water and set aside.
3. In another medium bowl set over the simmering water, whisk the yolks of the eggs with the 2/3 cup of sugar, rum, and water for about 3 minutes until the mixture is thick, like runny mayonnaise. (You can also use a handheld electric mixer.)
4. Remove the bowl from heat and place the bowl of whipped egg yolks in the bowl of ice water. Beat gently until cool and thick. Remove the bowl from the ice, and fold the chocolate mixture into the egg yolks.
5. In a separate bowl, beat the egg whites with the salt until frothy. Continue to beat until they start to hold their shape. Whip in the 1 tablespoon of sugar and continue to beat until thick and shiny, but not completely stiff, then add the vanilla.
6. 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.
7. Transfer the mousse to a serving bowl or divide into 6 or 8 serving dishes, and refrigerate for at least 4 hours, until firm.