In a small saucepan whisk together granulated sugar and cornstarch and add water and egg, whisking until smooth. Bring mixture to a boil over moderate heat, whisking and simmer, whisking vigorously, 1 minute. Remove pan from heat and add chocolate and eau-de-vie or liqueur, stirring until chocolate is melted. Transfer mixture to a metal bowl set in a bowl of ice and cold water and beat until cold and lightened in color. In a bowl, beat cream until it just holds stiff peaks and fold into chocolate mixture gently, but thoroughly. Chill mousse, covered, at least 30 minutes and up to 2 days. Serve with whipped cream and chocolate curls.