Scatter the caster sugar on the bottom of a heavy-based saucepan. Make sure to create an even layer. Break the cinnamon sticks in half to release their flavour and add them to the sugar. Cook on a medium heat until it forms a caramel.
Add the milk and the cream and bring the mixture to the boil. Be very careful while pouring the milk and cream as the caramel may splatter.
Beat the egg yolks together with the corn flour in a large bowl. Gradually, pour the caramel milk over the egg yolks while whisking constantly. Put the mixture back into the saucepan and cook at medium heat.
Use a wooden spoon to constantly stir the custard until it thickens. Make sure to scrape the bottom while stirring in order to prevent the custard from sticking to the saucepan.
Sieve the custard over a bowl.