In a food processor, combine the flour, sifted icing sugar, and a pinch of salt. Pulse a few times to blend. Add the chilled butter cubes and pulse until the mixture forms fine crumbs resembling sand.

Lightly beat the egg in a small bowl, then pour it into the food processor. Pulse gradually until the dough begins to clump together, being cautious not to overmix.

Turn the dough out onto a clean surface and shape it into a compact disc. Roll the dough between two sheets of parchment paper to a thickness of about 4 mm (1/6 inch). Transfer the rolled dough, still between the paper, onto a tray and refrigerate for a minimum of 1 hour, or up to 24 hours to rest.

Remove the top sheet of parchment. Using a round or fluted cookie cutter that's roughly 1.5 to 2 cm wider than your muffin pan openings, cut out circles of dough. Carefully fit each dough round into a muffin pan cavity, gently pressing into the corners to eliminate any trapped air and ensure full contact.

For best results, use a perforated muffin pan to allow even airflow and reduce shrinking during baking. Chill the prepared shells in the refrigerator for at least 2 hours, or overnight.

Preheat the oven to 160°C (325°F). While the oven heats, prick the base of each tart shell with a fork and freeze the pan to help the pastry hold its shape.

Line each tart shell with crumpled baking paper and fill with pie weights, rice, or dried beans. Bake for 15 minutes. Carefully remove the weights and paper, then bake for another 15 minutes or until the pastry is golden and dry to the touch. Let the shells cool completely before filling.

In a small saucepan, whisk together the egg yolks, sugar, and lemon juice. Place the pot over very low heat, stirring constantly until the mixture begins to slightly thicken—this should take about 3 to 5 minutes.

Gradually incorporate the butter, a few cubes at a time, stirring continuously and allowing each addition to fully melt before adding more. Stir in the lemon zest, if using.

Continue cooking the curd over low heat, whisking without pause, for 5 to 15 minutes until it thickens. To test, dip a spoon into the curd and swipe a line through the back with your finger. If the line holds without the curd running, it’s done. If the curd covers the line, it needs more time.

While the lemon curd is still warm, carefully spoon or pour it into the cooled tartlet shells. Lightly tap each tartlet on the counter to release any air bubbles trapped in the curd. Place the filled tartlets in the refrigerator to chill and firm up for 2 to 3 hours before serving.