Make the Pastry Dough: In a large bowl, sift the plain flour and icing sugar. Add the cold, cubed butter. Using your fingertips, rub the butter into the flour until the mixture resembles fine breadcrumbs. Alternatively, pulse it in a food processor. Mix the egg yolk with 2 tablespoons of cold water, then add it to the flour mixture. Bring the dough together with your hands (or a knife) until it just forms a ball. Add the final tablespoon of water only if it seems too dry.

Chill the Pastry: Wrap the dough in cling film and flatten it into a disc. Let it rest in the fridge for at least 30 minutes. This step is crucial for preventing the pastry from shrinking when baked.

Prepare the Tins and Bake Blind: Preheat your oven to 180°C (160°C fan). Lightly grease a 12-hole muffin tin or individual tartlet tins. On a floured surface, roll out the chilled pastry to about 3mm thick. Cut out 12 circles, slightly larger than your tin holes. Gently press the pastry circles into the tin, ensuring there are no air gaps. Prick the bases with a fork and chill again for another 15-20 minutes.

Bake the Pastry Cases: Line each pastry case with a small piece of baking parchment and fill with baking beans or uncooked rice. Bake for 15 minutes. Carefully remove the paper and beans, then return the cases to the oven for another 5-7 minutes, or until they are a pale golden colour and feel dry to the touch. This process of blind baking ensures a crisp base. Set aside to cool.

Start the Lemon Curd: While the pastry cools, make the filling. In a heatproof bowl set over a saucepan of gently simmering water (a bain-marie), add the lemon zest, lemon juice, caster sugar, and butter. Stir occasionally until the butter has completely melted.

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

Cook Until Silky: Return the bowl to the pan over a low heat. Stir continuously with a wooden spoon or spatula for about 8-10 minutes. I find that scraping the bottom and sides is key. The curd is ready when it has thickened enough to coat the back of the spoon. Don’t let it boil!

Fill and Final Bake: Pour the warm lemon curd into the cooled pastry cases. Tap the tin gently on the worktop to settle the filling. Bake for a final 5-7 minutes, just until the curd is set but still has a slight wobble in the centre.

Cool and Serve: Let the lemon curd tartlets cool completely in the tin before carefully removing them. They are delicate, so be gentle! Serve at room temperature or slightly chilled.