Prepare the Tart Shells:

1. In a large mixing bowl, combine the flour and sugar. Cut in the cold butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.
2. Add the egg yolk and mix gently. Gradually add cold water, 1 tablespoon at a time, until the dough just comes together.
3. Shape the dough into a disk, wrap in plastic wrap, and refrigerate for 30 minutes.
4. Preheat the oven to 350°F (175°C). Roll out the dough on a lightly floured surface to about 1/8-inch thick.
5. Cut out circles and press them into a mini tart pan or muffin tin. Trim any excess dough.
6. Prick the bottom of each shell with a fork and bake for 12–15 minutes or until golden brown. Let cool completely.

Make the Lemon Curd:

1. In a heatproof bowl, whisk together lemon juice, zest, sugar, and eggs. Place the bowl over a saucepan of simmering water, ensuring the bowl doesn’t touch the water.
2. Whisk continuously until the mixture thickens, about 8–10 minutes.
3. Remove from heat and whisk in butter, a few pieces at a time, until smooth. Let the curd cool to room temperature.

Assemble the Tartlets:

1. Spoon the lemon curd into the cooled tart shells, filling each to the top.
2. Chill the tartlets in the refrigerator for at least 1 hour before serving to set the curd.