Dice the chocolate bar into very small pieces and set aside.
In a medium pot, heat the milk over medium heat until it just reaches a boil, then reduce heat.
In a separate pot over low heat, melt the chocolate gently.
Slowly stir a few spoonfuls of the hot milk into the melted chocolate, forming a thick, smooth sauce.
Gradually add the remaining milk, stirring constantly.
Stir in the sugar and cinnamon until fully dissolved.
In a bowl, beat the egg yolks. Temper them with a spoonful of the hot chocolate mixture.
Slowly pour the tempered yolks into the pot while whisking aggressively.
Continue stirring over low heat for several minutes until the chocolate thickens to a custard-like consistency. Do not boil.
Serve immediately. Garnish with unsweetened whipped cream and shaved dark chocolate if desired.
