Make the Pastry: In a large bowl, sift the plain flour and icing sugar together. Add the cold, cubed butter. Using your fingertips, rub the butter into the flour until the mixture resembles fine breadcrumbs. Stir in the beaten egg with a knife, then add 1-2 tablespoons of cold water, mixing until the dough just comes together. Don't overwork it!

Chill the Dough: Tip the dough onto a lightly floured surface, gently shape it into a flat disc, wrap it in cling film, and chill in the fridge for at least 1 hour. This resting period is crucial for preventing shrinkage during baking.

Roll and Line the Tins: Preheat your oven to 180°C (160°C Fan). Lightly grease a 12-hole tartlet or muffin tin. On a floured surface, roll the chilled pastry out to about 3mm thick. Use a round cutter (around 10cm) to cut out 12 discs. Gently press the pastry discs into the tin, trimming any excess from the top.

Blind Bake the Cases: Prick the base of each pastry case with a fork. Line each one with a small piece of baking parchment and fill with baking beans or uncooked rice. Bake for 15 minutes. Carefully remove the parchment and beans, then return the cases to the oven for another 5-7 minutes, or until they are pale golden and look dry. Set aside to cool slightly.

Prepare the Lemon Curd: While the pastry bakes, make the filling. Put the lemon zest, lemon juice, caster sugar, and cubed butter into a heatproof bowl. Set the bowl over a saucepan of gently simmering water, creating a bain-marie. Stir until the butter has melted and the sugar has dissolved completely.

Cook the Curd: In a separate bowl, lightly whisk the 3 eggs and 1 egg yolk. Gradually pour the warm lemon mixture into the eggs, whisking constantly to prevent scrambling. Pour the entire mixture back into the heatproof bowl.

Thicken the Curd: Return the bowl to the pan over a gentle heat and cook, stirring continuously with a wooden spoon or spatula for about 8-10 minutes. What works best for me is to keep stirring until the curd is thick enough to coat the back of the spoon. Don't let it boil!

Fill and Final Bake: Strain the curd through a fine-mesh sieve into a jug for a perfectly smooth texture. Carefully pour the warm lemon curd into the pre-baked pastry cases. Bake for a final 5-8 minutes, just until the filling is set but still has a slight wobble in the centre.

Cool and Serve: Let the lemon tartlets cool completely in the tin before carefully removing them. They are delicate, so take your time. Dust with a little icing sugar before serving, if you like.