Place the lemon pieces, cinnamon and vanilla pod into a pan with 100ml water and 275ml milk and bring to a boil and then take off the heat and leave to infuse. Meanwhile, beat together the eggs, egg yolks and sugar with an electric whisk until the mixture is pale and thick. Gradually, pour the hot infused milk over the eggs and beat continuously until combined. In the small bowl, mix the flour, cornflour and with a splash of milk until you have a smooth paste and then pour into the egg mix and beat together.
Pour everything back into a clean pan and bring to a simmer, whisking until the mixture thickens. Remove the cinnamon and lemon, pass through a sieve into a jug, cover the surface of the custard with cling film and leave to cool. Heat the oven to 220C/fan 200C/gas 7 and put a baking sheet in the oven to heat up. Roll out the puff pastry on a clean work surface lightly dusted with flour, icing sugar and ground cinnamon. Cut the pastry in half and lay one sheet on top of the other. Roll the pastry sheets up like a Swiss roll and cut the roll into twelve slices about 1 cm thick. It is easier if the pastry is chilled so chill for a few minutes before slicing if it is proving difficult.
Lay each of the pastry slices flat on the work surface and roll them out into 10cm discs with a rolling pin. Press a pastry disc into each of the wells of a buttered muffin tray and divide the custard evenly between the pastry cases. Bake the tarts for 18-20 minutes on the preheated baking sheet, or until the custard has puffed up and is pale golden-brown, and the pastry is crisp and golden-brown. Allow to cool in the tin. Before serving dust with some cinnamon and icing sugar
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