Prepare the shortcrust pastry: pile the flour, sieved with a pinch of salt, up on your work surface and create a “well” in it. In the centre put the softened butter (you must take it out of the fridge half an hour beforehand) cut in pieces, the egg and the extra yolk, and the grated peel of the orange. Delicately mix the ingredients, first with a fork and then with your hands (if possible “cold”), without working the dough too much so that the pastry will be crumbly. Shape it into a ball, sprinkle lightly with flour and put it in the fridge, wrapped in cling film, for at least half an hour.
In the meanwhile prepare the pastry cream: boil the milk and then let it cool. In a small, deep saucepan beat the egg yolks and the sugar well; add the flour a little at a time, stirring carefully, then the juice of ½ an orange and, very gradually, the cold milk. Put the saucepan back on the stove and keep stirring continuously; when the pastry cream thickens remove the pan from the heat for a moment and stir well, then put it back on the stove and just before it comes to the boil turn the heat off. Cover it with cling film and set it aside.
Take the shortcrust pastry out of the fridge and, using a rolling pin, roll it out into a round shape roughly 3 cm larger in diameter than the tart tin. Use your hands to stretch the pastry out in the tin (which you have buttered and floured), lining the bottom and around the side. Prick it here and there with a fork and bake in a preheated oven, at 180 °C, for 15-20 minutes. Let it cool down, remove it from the tin, place it on a serving dish, pour the pastry cream into it and level the surface. Peel both the oranges completely (without leaving the white pith) and cut them into slices 5 mm thick. Place them delicately on top of the pastry cream, slightly overlapping, and sprinkle with the cinnamon. Put the sugar and water in a small saucepan; stir with a whisk until the sugar dissolves and then start heating the pan. Cook without stirring, until the sugar becomes caramelised and turns pale brown. Pour the caramel over the slices of orange, leave it to cool and…. the cake is ready!