The pastry can be made well in advance.
For the pastry, cream the butter and sugar together until light and aerated (best done with an electric mixer). Add the egg yolks one by one, and beat until well amalgamated. Add the sifted flour and salt and very gently knead into a dough without overworking. Shape into a slightly flattened ball, wrap in clingfilm and refrigerate for 1 hour.
Roll out the pastry on a lightly floured surface to a disc of at least 27cm (10¾in) diameter, then carefully ease it into a 24cm (9½in) tart tin, making sure it fits into the corners and hangs over the edge all the way round. Do not cut off this overhang, but use any surplus to make certain any holes are repaired. Refrigerate the case for 30 minutes.
Preheat the oven to 180°C (350°F, Gas Mark 4). Line the tart case with greaseproof paper or foil and dried or baking beans, and bake for 25 minutes. Remove the beans and paper and return to the oven for 5 minutes.
Beat the egg and milk together and brush inside the tart case the minute it comes out of the oven. Return the case to the oven for 3–4 minutes. Allow to cool a little.
For the filling, very finely grate the zest of the oranges into a bowl, then strain over the well-squeezed juice. Whisk the eggs, the extra yolk and the sugar together in a stand mixer or with hand-held electric beaters until the sugar has dissolved and the mix is smooth. Pour in the cream and mix well before stirring in the juice and zest.
Turn the oven down to 150°C (300°F, Gas Mark 2). Place the tart tin on the middle shelf, one-third of the way out of the oven. Stir the filling if you have let it rest, then carefully pour it into the case and slide it very carefully into the oven. It will take about 40 minutes to cook. The surface should not colour: if it threatens to do so, cover it with foil. To test, give the tin a gentle nudge back and forth – there should be no sign of liquid movement beneath the surface of the tart.
Allow the tart to cool a little before cutting off the overhang with a serrated knife and gently lifting it off. Transfer the tart to a plate once it is completely cool, then refrigerate. Dust the tart with a sprinkling of icing sugar and serve chilled. It needs no further accompaniment.
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