Prepare the Tart Dough

1. Mix Dry Ingredients: In a food processor, pulse together flour, icing sugar, and salt.
2. Add Butter: Add cold butter cubes and pulse until the mixture resembles fine crumbs.
3. Incorporate Egg: Whisk egg in a bowl and slowly add it to the food processor while pulsing until the dough starts coming together.
4. Chill Dough: Form the dough into a ball, roll it between two sheets of baking paper to 4mm (⅙ inch) thick, then refrigerate for at least 1 hour (or up to 24 hours).

Prepare & Bake Tart Shells

5. Cut Tart Shells: Remove baking paper and cut out disks of dough using a round or fluted cutter 1.5–2 cm wider than the muffin pan openings.
6. Press Into Pan: Gently press dough into a muffin pan, ensuring it touches all sides to prevent air bubbles.
7. Chill Again: Refrigerate for at least 2 hours, preferably overnight.
8. Preheat Oven: Preheat to 160°C (325°F) and prick the tartlet bottoms with a fork.
9. Blind Bake:
   • Place the muffin pan in the freezer while the oven preheats.
   • Line each tartlet with baking paper and fill with baking weights, rice, or beans.
   • Bake for 15 minutes, remove weights, and bake for another 15 minutes until golden.
   • Set aside to cool completely.

Make the Lemon Curd

10. Cook the Base: In a saucepan over low heat, whisk together egg yolks, sugar, and lemon juice for 3-5 minutes, until slightly thickened.
11. Incorporate Butter: Gradually whisk in butter cubes, ensuring each addition melts before adding more. Stir in lemon zest (if using).
12. Thicken the Curd: Continue cooking on low heat for 5-15 minutes, whisking constantly, until the curd thickens.
13. Check Readiness: Dip the back of a spoon into the curd and run your finger through it. If the curd stays separated, it’s ready. If it flows back together, cook longer.

Assemble the Tartlets

14. Fill the Shells: While warm, pour the lemon curd into the cooled tartlet shells.
15. Remove Air Bubbles: Gently tap the tartlets against a hard surface.
16. Chill & Set: Refrigerate for at least 2-3 hours before serving.