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 (15-20 minutes).
Meanwhile, with an electric mixer on medium-high speed, beat the egg white, just until soft peaks form. (I did this by hand, with a balloon whisk, since it was such a small amount of whites). Gradually add sugar and beat until stiff peaks form.
Whisk sour cream and vanilla into chocolate and vanilla into chocolate mixture. In two batches, whisk the egg-white mixture into the chocolate mixture until just blended. Transfer mousse to small serving dishes or stemmed glasses.
Cover dishes ightly with plastic wrap and refrigerate until thoroughly chilled and set, at least 1 hour or up to 1 day. Top each mousse with fresh whipped cream before serving.
Makes about 6, 1/3 cup servings.