Break up the chocolate into a small, heavy saucepan. Dissolve the coffee or espresso in the boiling water and pour it over the chocolate. Place over low heat and stir occasionally with a small wire whisk until mixture is smooth. Remove from heat and set aside for about 5 minutes.
In the bowl of an electric mixer at high speed, beat the yolks for 3 to 4 minutes until pale lemon-colored. With a spatula add the chocolate mixture and mix until smooth. Do not over mix.
Add the salt to the egg whites until they hold a definite shape but are not dry. Gently fold whites into chocolate mixture in thirds. I do this by adding the whites to the chocolate twice and then adding the chocolate mixture back to the bowl where the whites are. Fold only until no whites show.
Gently pour into serving glasses or bowls, filling ¾ of the full capacity. Cover with plastic wrap and refrigerate for 3 to 6 hours. You may refrigerate it for as long as 24 hours, though the texture will become more spongy and less creamy. It's good either way.