Heat in a medium heavy saucepan, stirring, until the chocolate is melted:
Meanwhile, beat well in a medium bowl:
Slowly whisk the hot milk into the egg yolk and sugar mixture. Return this mixture to the saucepan, set over medium-low heat, and stir the sauce gently but constantly, until the custard reaches 170° to 175°F on an instant-read thermometer and coats the back of a spoon. Immediately strain the sauce into a bowl. Stir in:
Serve hot or cold.