Add enough water to a large saucepan to come 1 inch up the side. Bring to a simmer over medium heat. Please the chocolate, salt, coffee, rum, and butter in a large mixing bowl and set over the simmering water.Don't get water in melting chocolate or it'll seize up. Stir occasionally until almost completely melted. Pull the mixture off the heat while a couple of small pieces are still floating around in there. Cool until just a bit warmer than body temperature (just touch the bowl).
Pour 1/4 cup of the cream into a metal 1-cup measure and sprinkle in the gelatin. Let soak or "bloom" for 10 minutes.
Meanwhile, add the remaining cream to the bowl of a stand mixer fitted with the whisk attachment and beat to medium peaks.
Carefully heat the bloomed gelatin by swirling the measuring cup over a low gas flame. Don't let it boil.
Stir the cream-gelatin mixture into the cooled chocolate. Stir one quarter of the whipped cream into the chocolate mixture to lighten it. Fold in the rest of the whipped cream in two doses. There may be streaks of whipped cream still visible in the chocolate, and that's okay. Whatever you do, don't over-mix the mousse.
Spoon into bowls, mugs, or martini glasses and chill for at least 1 hour. Garnish with fruit, if desired, and serve. If mousses are to be refrigerated overnight, chill for 1 hour and then cover each tightly with plastic wrap.