Combine first 3 ingredients in a medium saucepan. Cook over low heat, stirring constantly, until chocolate melts; remove from heat.
Beat egg yolks well, and stir in 1/4 cup whipping cream. Whisk about one-fourth of chocolate mixture into egg mixture; whisk back into remaining chocolate mixture. Cook, stirring constantly, over medium-low heat until a thermometer registers 160°. Remove from heat; stir in orange liqueur and vanilla. Cool to room temperature, stirring occasionally.
Beat remaining 3/4 cup whipping cream until soft peaks form. Fold about one-fourth of whipped cream into cooled chocolate mixture; fold in remaining whipped cream.
Spoon mousse into 8 goblets. Cover and chill thoroughly. Garnish, if desired.