In a saucepan, gently dissolve the caster sugar in 250ml of water along with the cinnamon stick and lemon zest.
Heat the sugar syrup until it begins to boil before reducing to a simmer for 3 minutes.
Place the syurp to one side and let it cool completely.
In a bowl, mix together the plain flour and cornflour.
Add some of the milk and stir until it forms a smooth paste.
Heat the reamining milk in a saucepan and bring to the boil, stirring frequently.
Once it has reached boiling point, slowly pour it into the bowl of flour, whisking well to make sure there are no lumps.
Once all of the milk has been incorporated, remove the cinnamon stick and lemon zest from the sugar syrup and whisk the sugar syrup into the milk and flour.
Next add the egg yolks, one by one, whisking well.
Finally, whisk in the whole egg followed by the seeds of the vanilla pod.