Bring 2" of water to a simmer in a medium saucepan. Set a bowl on top and add the chocolate, egg yolks, coffee, sugar, butter and salt. Stir together until melted and mixture has thickened and increased in volume. Set aside and cool to room temperature.
In a separate bowl, use an electric hand mixer to whip the eggs to stiff peaks.
In another bowl, whip 1 cup of the cream to stiff peaks.
Fold ½ of the egg whites gently into the chocolate, then half of the cream. Repeat with the remaining egg whites and cream.
Divide the mousse between serving dishes and refrigerate until firm.
Whip the remaining cream with the powdered sugar to soft peaks. Top the mousse, if desired.