To make the custard, whisk 30ml of the milk with the plain flour in a medium sized bowl.
In a saucepan heat the remaining 120ml of milk with the vanilla and cinnamon to boiling point.
In another saucepan over a low heat, heat the water and sugar together until it reaches 100c (use a sugar thermometer for this). Slowly pour the hot milk over the flour mixture whilst whisking continuously.
Once the sugar syrup has reached 100c, slowly pour this into the bowl of hot milk, whisking continuously still.
Leave aside to cool for 10 minutes before adding the egg yolks (if you add them to the hot mixture they will scramble.) Whisk in the yolks, then strain through a fine sieve and into a jug. Cover with cling film while you get on with the pastry.
Preheat the oven to 240c, or higher if it will go.
Lightly dust a work surface with plain flour. Unwrap the sheet of pastry and remove the paper, then roll it up again tightly in exactly the same way.
Slice the pastry log into 12 even sized rounds and roll them out into thin circles, placing each one into a muffin case hole.
Use your fingers to push the pastry out to cover the bottom and sides of each hole evenly.
Divide the custard between each muffin hole, then place into the top third of the hot oven for 15 minutes. The pastry should be crisp and the custard blistered and bubbling.
Leave to cool in the tin for 15 minutes, then carefully remove to cool completely on a wire rack.
Before serving dust with icing sugar, adding a pinch of cinnamon if desired.