First make the pastry. Sift the flour and salt together, then mix in the sugar. Rub the butter into the flour with your fingertips until the mixture has the texture of fine breadcrumbs. Make a well in the centre, add the egg yolk or a little iced water and mix quickly to a dough with a knife. Add more water gradually, if necessary. Bring the dough together with your fingers, and knead gently for 1–2 minutes on a lightly floured board. Shape the pastry into a ball, wrap in clingfilm and refrigerate for at least 30 minutes to allow it to relax.
Make the custard topping while the tart is in the oven. Beat the eggs with the rest of the measured sugar, making sure the sugar has dissolved. Whisk in the cream gradually. Pour the mixture over the tart and bake for a further 12–15 minutes, or until the custard is set. Sprinkle caster sugar over the tart and leave to cool. Serve cold, but not chilled.