In heavy 2-quart saucepan, mix 2/3 cup sugar, the cocoa, cornstarch and salt. Stir in milk with whisk until blended. Beat in egg yolks with whisk until blended. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 3 minutes. Remove from heat. Add chopped chocolate, liqueur, vodka and vanilla; stir until smooth.