Warm the milk and sugar over a medium-low heat until it is just boiling. Lightly beat the egg yolk in a bowl and gradually add the milk, whisking it in as you add to ensure it combines well and doesn't cook the egg.

Return the mixture to the pan then add the chocolate and melt in the heat of the milk mixture, stirring constantly. Add the cinnamon and ½ tablespoon ginger and mix through.

Warm the mixture for a couple minutes back on a low stovetop heat, without boiling, to thicken the mixture slightly. Then stir in butter and divide it between two small glasses. Refrigerate before serving, topped with additional small pieces of crystalized ginger.