In pan over low heat or in double boiler, over hot, not boiling water, heat chocolate and corn syrup until chocolate is melted and mixture is smooth. Stir occasionally with spatula. Cover and refrigerate 30 minutes or until mixture is smooth. Stir in vanilla.
Stir in vanilla. In pan over medium low heat, heat milk until very hot and small bubbles form around edge. Do not boil. Meanwhile, in small bowl, with mixer at medium low speed, beat heavy cream and chocolate mixture until soft peaks form. To serve, pour milk into warm coffee pot. Carefully spoon whipped cream mixture equally into 8 warmed decorative coffee cups. Gently pour hot milk mixture into cups. Serves 8
Source; My Old Recipes