Steps

1. Break up the chocolate into a small, heavy saucepan. Dissolve the coffee 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 to cool for about 5 minutes.
2. In the small bowl of an electric mixer at high speed, beat the egg yolks for 3 to 4 minutes until they turn a pale lemon color. Reduce the speed and gradually add the slightly warm chocolate mixture, scraping the bowl with a rubber spatula and beating only until smooth. Remove from mixer.
3. Add the salt to the whites and beat until they hold a definite shape but are not dry. Without being too thorough, gently fold about one-quarter of the beaten whites into the chocolate mixture, then fold in a second quarter, and finally fold the chocolate into the remaining whites, folding only until no whites show.
4. Gently transfer the mousse to a wide pitcher and pour it into six large dessert or wine glasses, each with about a 9-ounce capacity. Don’t fill the glasses too full; leave generous headroom on each. (This mousse must be prepared in individual portions. It just won’t work if it is all done in one large container.)
5. Cover with plastic wrap or aluminum foil and refrigerate 3 to 6 hours. (The mousse may stand longer, up to a day. The texture will become more spongy and less creamy. Delicious both ways.)
6. Serve as is, or topped with Mocha Cream.