Preheat the oven to 200°C (180°C fan) or 400°F. Line a 12-hole muffin tin with paper cases or lightly grease it. The heat should feel intense when you open the door later.

Make the lemon curd: In a heatproof bowl set over a pan of simmering water (make sure the bowl doesn't touch the water), whisk together the lemon juice, lemon zest, caster sugar, eggs, egg yolk, and a pinch of salt. Stir constantly with a wooden spoon or silicone spatula. After about 8–10 minutes, the mixture will thicken noticeably and coat the back of the spoon — you'll see it turn from a thin, watery liquid to a pale, creamy yellow. The surface should look glossy and feel heavy on the spoon.

Finish the curd: Remove the bowl from the heat and whisk in the cubed butter, one piece at a time, until each piece is fully melted and incorporated. The curd will become silky and smooth. Pour it through a fine-mesh sieve into a clean bowl to catch any cooked egg bits. Let it cool slightly — you should smell a rich, buttery lemon fragrance. Cover with cling film pressed directly onto the surface to stop a skin forming, then chill in the fridge.

Prepare the pastry: On a lightly floured surface, unroll the puff pastry sheet. Using a 9cm round cutter (or a small bowl as a guide), cut out 12 rounds. Gently press each round into the muffin tin holes, pricking the bases a few times with a fork. This stops the pastry from puffing up too much — you'll see tiny steam holes appear as it bakes. Brush the edges with the beaten egg for a golden shine.

Blind bake the pastry: Place a small square of baking paper inside each pastry case and fill with baking beans or dried rice. Bake for 10 minutes, then remove the paper and beans. Return to the oven for another 3–5 minutes, until the pastry is pale golden and feels crisp to the touch. The smell of baked butter will fill your kitchen.

Fill the tarts: Spoon the chilled lemon curd into each pastry case, filling them nearly to the top. The curd should sit level with the pastry rim. Return to the oven and bake for 5–7 minutes, until the curd is just set with a slight wobble in the centre — it should look matte rather than shiny when done.

Cool and serve: Let the tarts cool in the tin for 5 minutes, then transfer to a wire rack. They'll firm up as they cool. Dust with icing sugar just before serving, if you like. The pastry should be flaky and golden, the curd set but still soft.