Very thinly roll the puff pastry in to a rectangle and then roll in to a sausage shape along the long edge. Divide into 20 equal pieces. Place on a floured tray in the fridge.
Peel the lemon and orange, ensure no white pith is present, if it is scrape it off. Put the peel, milk and cinnamon stick in a pan and heat to simmering. Remove from the heat and leave to infuse for 30 mins.
Place the egg yolks and flour in a pan and add enough of the milk to make the mixture in to a paste, add the remaining milk and whisk to combine.
Place the sugar and 100g water in a pan and heat to 108C. Once at this temp add it to the remaining ingredients. Cook over a medium heat until the mixture thickens. Keep it moving as it will catch on the base of the pan. Once thickened taste to ensure that there is not a raw flour taste.
Once cooked pour the mixture in to a piping bag.
Remove the pastry from the fridge and press the pastry with the palm of your hand and then roll in to very thin circles. Place on a tray in the fridge.
When chilled, remove from the fridge and press in to the muffin moulds. Pipe the mixture in to each mould, about half way up.
Bake at 230C for 15 mins until the tops of the tarts are slightly blistered and blackened.