First, make the pastry. Put the flour, butter, icing sugar and 1/4 tsp salt in a food processor and pulse until you can’t feel any lumps of butter. Add the egg yolk, vanilla, orange zest and 2 tsp cold water, and pulse again until the pastry starts to clump together in pieces. Tip onto your surface and gently bring together into a ball. Wrap in cling film and chill for 30 mins.
Roll out the pastry on a floured surface to line a 23cm round tart tin, about 3cm deep. Pinch any overhanging excess back down the outside of the tin (to trim after baking), prick the base with a fork and chill for another 30 mins.
Heat oven to 200C/180C fan/gas 6. Line the pastry with a big sheet of scrunched up baking parchment, then fill with baking beans or rice. Bake for 15 mins, then remove the paper and beans, and bake for another 5-15 mins until golden brown and crisp. Let the pastry cool for 5 mins, then carefully trim the excess pastry with a small, sharp knife. Beat the egg white with a fork, then brush all over the inside and cut edge of the pastry and bake for a further 2 mins – this helps keep the pastry crisp when the wet filling is added.
Remove the pastry case from the oven and lower the heat to 160C/140C fan/gas 3. For the filling, whisk together the whole eggs and yolks, then sieve into a larger mixing bowl. Whisk in the caster sugar, crème fraîche, orange zest and juice, and lemon juice. Lift the pastry case back into the oven and place on the middle shelf. Using a jug, pour the filling into the tart. Bake for 35 mins until there is just a slight wobble in the centre when you tap the tin. Cool to room temperature, then chill for 1 hr. LIghtly dust with icing sugar, then serve.