Make the filling whilst the pastry is baking. Line a medium saucepan with butter - this'll prevent the milk mixture from burning around the edges of the saucepan. Add the milk, cream, and cinnamon stick to the medium saucepan and bring to a light simmer.
In a separate medium-sized bowl add the sugar, flour, cornstarch, vanilla essence, egg, and salt together and mix well.
Remove the milk mixture from the heat once it has started boiling and discard the cinnamon stick.
Temper the egg mixture by gradually pouring the warm milk mixture in to the egg mixture, stirring vigorously. Bring the milk mixture back to the heat and allow to simmer until thickened whilst stirring. Once thickened, add the butter and stir until combined.
By now the pastry should be done baking. Remove the baking paper/rice and pour the warm filling into the baked pastry shell & smooth the surface (I leave it in the tart tin until fully set).
Sprinkle over the cinnamon and place int the fridge to set overnight.