1. Place chocolate, half-and-half, and salt in a double boiler or a heatproof bowl; cook over simmering water until chocolate is melted, stirring frequently (about 5 minutes). Set aside to cool to room temperature (1520 minutes).
2. Meanwhile, with an electric mixer on medium-high speed, beat the egg white in a medium bowl, just until soft peaks form. Gradually add the sugar and beat until stiff peaks form.
3. Whisk sour cream and vanilla into chocolate mixture. In two batches, whisk the egg-white mixture into the chocolate mixture until just blended. Transfer mousse to 3 serving dishes (about 6- to 8-ounce capacity each, such as stemmed glasses, teacups, or small dessert bowls). Cover dishes lightly with plastic wrap, and refrigerate until thoroughly chilled and set, at least 1 hour or up to 1 day. Before serving, top each mousse with whipped cream and chocolate curls, if desired.