Sift the icing sugar in a mixing bowl, add the softened butter and mix them together without creaming them. Add the egg yolks, salt, lemon zest and sifted flour. Combine all the ingredients together, using your fingertips, until you get a soft but not sticky dough.
Place the dough on a piece of cling film, flatten it to about 1 cm and leave to rest in the fridge for about 30 minutes.
Take the dough out of the fridge, take it out of the cling film, place it on a lightly floured surface and leave it to rest for 2 minutes. Using a well floured rolling pin, roll the dough until is about 5 mm thick. Wind the dough on the roller pin then unwind it over the flan. Gently push the pastry trough the edges, then cut off the excess pastry. Place the pastry in the fridge for 15 minutes.
Line the pastry with parchment paper and add the baking beans. Bake the pastry in the oven, at 190 degrees Celsius, for 20 minutes. remove the baking beans and parchment paper, reduce the oven temperature to 180 degrees Celsius and place the pastry back in the oven for another 15 minutes.