Combine first 4 ingredients in a small saucepan. Stir in cream. Cook over low heat, stirring constantly, just until chocolate is melted.
Whisk 1/2 cup warm chocolate mixture into egg yolk in a small bowl; add to remaining warm chocolate mixture. Increase heat to medium-low, and cook 20 minutes, whisking constantly, just until sauce comes to a boil.
Remove from heat; stir in vanilla. Pour into a serving bowl.
Note: Leftover sauce can be refrigerated and served as pudding.