Make the pastry dough:
In a food processor, combine flour, icing sugar, and salt. Add cold butter cubes and pulse until the mixture resembles fine breadcrumbs. Add the egg and pulse just until the dough comes together.

Chill the dough:
Shape the dough into a disc, wrap in plastic wrap, and chill for at least 1 hour (or overnight).

Form the tartlet shells:
Roll the dough to 3–4mm thickness. Cut rounds and press into a muffin pan. Prick bottoms with a fork and chill for another hour.

Blind bake the tart shells:
Preheat oven to 325°F (160°C). Line each tartlet with parchment and fill with baking weights. Bake for 15 minutes, remove weights, and bake another 10–15 minutes or until golden. Let cool completely.

Make the lemon curd:
In a small pot over low heat, whisk together the egg yolks, sugar, and lemon juice. Cook, stirring constantly, until the mixture thickens (about 5–10 minutes). Gradually whisk in butter until fully melted. Add lemon zest (if using).

Fill the tartlets:
Pour the warm curd into the cooled tartlet shells. Gently tap the tray to smooth the tops and release any air bubbles.

Chill to set:
Refrigerate the filled tartlets for at least 2–3 hours, or until the curd is fully set.