Heat first 3 ingredients in a medium saucepan over medium-low heat until hot.
Whisk egg yolks until smooth. Gradually whisk 1/2 cup hot milk mixture into egg yolks; gradually whisk egg mixture into hot milk mixture. Cook over medium heat, stirring constantly, 10 minutes or until slightly thickened. (Thermometer should register 160°.) Add chocolate and coffee, stirring until smooth and well blended.
Pour chocolate mixture into 8 espresso cups; refrigerate 2 hours or overnight. Garnish, if desired.