Beat the egg whites with the salt until very firm and set aside. Whip the heavy cream with sugar. Slowly incorporate the espresso into the whipped cream. Carefully fold the espresso cream into the egg whites.

Spoon the coffee mousse into 6 bowls or cups and refrigerate for at least 1 hour before serving.

Note: I love to serve this mousse after dinner (instead of offering coffee) with fresh baby madeleines.