Make the pastry crust: In a large bowl, rub the 225g flour and 150g cold butter together with your fingertips until it resembles coarse, pale yellow breadcrumbs with a few pea-sized butter lumps still visible—you should feel the cool, crumbly texture between your fingers. Stir in the 35g icing sugar, then add the egg yolk and 2 tablespoons of ice-cold water. Bring together with a fork until the dough just holds when pinched; if it feels dry, add the final tablespoon of water. Turn out onto a lightly floured surface, knead gently twice to form a disc, wrap in cling film, and chill for 30 minutes until the dough feels firm and cool to the touch.

Blind-bake the crust: Preheat your oven to 190°C (170°C fan). Roll the chilled dough out on a floured surface to about 3mm thick—the dough should smell sweet and buttery. Line a 23cm loose-bottomed tart tin, pressing into the fluted edges. Prick the base all over with a fork, line with parchment paper, and fill with baking beans or dried rice. Bake for 15 minutes until the edges are just starting to turn a pale, biscuity gold. Remove the paper and beans, then bake for a further 8–10 minutes until the base is dry, lightly golden, and smells deeply of toasted shortbread. Set aside to cool completely on a wire rack.

Prepare the lemon filling: In a medium saucepan, whisk together 150g caster sugar, 40g cornflour, and 1/4 teaspoon salt. Gradually pour in 480ml cold water, whisking constantly until the mixture is smooth and pale—you should hear the gentle scrape of the whisk against the pan. Place over a medium heat and cook, stirring without stopping, for 5–7 minutes. You'll feel the mixture thicken suddenly; it will go from a thin, milky liquid to a thick, glossy paste that bubbles sluggishly. Remove from the heat immediately once you see the first large, slow bubble break the surface.

Finish the curd: In a separate small bowl, lightly beat the 4 egg yolks. Gradually whisk about a third of the hot cornflour mixture into the yolks to temper them—this prevents scrambling. Pour the yolk mixture back into the saucepan, whisking vigorously. Return to a low heat and cook for 2 minutes, stirring constantly, until the filling is thick, smooth, and coats the back of a spoon thickly; it should look glossy and feel heavy on the whisk. Remove from the heat, stir in the lemon juice, zest, and 60g diced butter until the butter melts and the filling turns a pale, sunshine yellow. Cover with a disc of damp greaseproof paper directly on the surface to prevent a skin forming, and let it cool for 15 minutes while you make the meringue.

Whip the meringue: In a clean, grease-free bowl (wipe the bowl with a little lemon juice on kitchen paper to be sure), place the 6 room-temperature egg whites and 1/2 teaspoon cream of tartar. In a small bowl, mix 250g caster sugar with 20g cornflour. Begin whisking the egg whites on medium speed until they become frothy and filled with tiny, even bubbles—the mixture should look like a white foam. Increase the speed to high and gradually add the sugar-cornflour mixture, a tablespoon at a time, whisking well between each addition. Continue whisking until the meringue is stiff, glossy, and holds a firm peak when you lift the whisk; the sugar should be fully dissolved—rub a little between your fingers to check it feels silky, not grainy. Beat in the vanilla extract.

Assemble and bake: Pour the warm lemon filling into the cooled pastry crust, spreading it evenly with a spatula—it should be thick and just hold its shape. Immediately spoon the meringue over the filling, starting at the edges and working inwards to seal the filling completely (this prevents weeping). Use the back of a spoon to create dramatic peaks and swirls; the meringue should look like soft, white clouds. Bake at 180°C (160°C fan) for 12–15 minutes, or until the peaks are a deep, toasted gold and you can smell the caramelised sugar. The meringue will feel firm to a gentle touch and the valleys should be pale ivory.

Cool completely: Place the pie on a wire rack and let it cool to room temperature—this takes about 2 hours. Do not rush this step. The filling needs to set fully, and you'll hear a faint crackling sound as the meringue settles. Once cool, transfer to the fridge for at least 1 hour before slicing to get clean, neat wedges. The first cut should reveal a crisp crust, a silky, firm lemon layer, and a pillowy meringue top.