In a small, heavy saucepan, heat the cream and chocolate over medium-low heat, stirring occasionally, until melted and smooth, about 5 minutes. Transfer to a medium bowl and place the bowl into a large bowl of ice water; let cool, stirring occasionally, for about 5 minutes.
Using an electric mixer with a whisk attachment, whip the egg whites on medium speed until foamy, about 1 minute. With the machine on, gradually whisk in the sugar and beat until stiff but still moist, 2 to 3 minutes. Transfer to a large bowl.
Transfer the chocolate-cream mixture to the mixer bowl and whip at medium speed until fluffy, about 2 minutes. Add one-quarter of the egg white mixture to the chocolate-cream mixture, folding with a rubber spatula just until no streaks remain. Fold in the remaining egg white mixture in the same manner. Divide the mousse among 8 cups or bowls and refrigerate for 30 minutes or up to overnight.