Gently heat the milk with the cinnamon and lemon rind for 10 minutes over medium-low heat. Do not boil.
Meanwhile, beat the egg yolks with the sugar till creamy. Beat in the cornflour. Continue beating till smooth and lump-free.
Remove the cinnamon and lemon rind from the milk. Gradually add the egg yolks to the milk, and beat with a whisk over low heat until mixture thickens, about 10 minutes. Bring to the boil and immediately remove from heat.
Pour the custard into individual serving dishes and chill in the fridge till set. Serve with a biscuit on top, such as a Marie biscuit or digestive.