Combine the cornflour, sugar and salt in a heatproof bowl. Slowly pour in milk, while stirring constantly and mix well. Float the bowl in a saucepan with lightly simmering water and heat the mixture for 15 to 20 minutes. Stir frequently and make sure the mixture does not stick to the bottom or sides of the bowl.
As soon as the mixture thickens, add the chocolate a little at a time. Stir until chocolate is melted and the pudding is smooth and thick. Remove the bowl from the heat and stir in vanilla extract and cinnamon.
Pour into a dessert bowl or individual dessert bowls. Let cool and cover with cling film. Chill in the fridge for at least an hour before serving.
A skin will form as the chocolate dessert chills in the fridge. If this is undesired, place the cling film in direct contact with the top of the dessert.