In a large saucepan combine 1 cup of the milk, the sugar, chocolate, and cinnamon. Cook and stir over medium-low heat until chocolate is melted. Gradually stir in remaining milk. Cook and stir until milk is very hot, almost boiling. Do not boil.
Gradually stir 1 cup of the hot milk mixture into eggs, then transfer entire mixture to saucepan. Cook and stir for 2 minutes over low heat. Remove from heat and stir in vanilla. Beat with a rotary beater until very frothy.
Pour hot chocolate into mugs and dollop with whipped cream. Makes six 8-ounce servings.