In a cold non-reactive bowl, whip the cream to soft peaks. Set aside.
Fill a saucepan about a third of the way with water, then bring to a simmer.
Chop the chocolate with a cook's knife. Put the chocolate in a medium-large heatproof bowl, and place over the water. Stir the chocolate occasionally until melted and smooth. Set the chocolate aside. Keep the pot of water hot, but not boiling.
In a small bowl, stir the espresso powder into the rum or water, to dissolve.
Put the egg, yolks, and pinch of salt in heatproof bowl. With a hand-held electric mixer whip the eggs until light, about 1 minute. Slowly add the sugar to the eggs, while continuing to beat. Add the rum mixture.
Set the bowl of eggs over the simmering water, increase the heat to bring the water to a rapid simmer. Whisk the egg mixture constantly over the heat, until pale yellow, very fluffy, and hot to the touch, 3 to 5 minutes. Remove from the heat and whip until cool and thick, and the mixture folds over on itself or "holds a ribbon".
Whisk about a quarter of the egg foam into the chocolate to lighten. With the whisk, briskly fold in the remaining foam, (the mixture may get very thick). Whisk in the whipped cream, taking care to keep the mixture light. Pour the chocolate mixture into 4 to 6 serving dishes and refrigerate for at least 2 hours.
Serve with cookies, if desired.
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