Prepare the Tartlet Dough: In a food processor, combine all-purpose flour, granulated sugar, and a pinch of salt. Add cold cubed unsalted butter and pulse until the mixture resembles coarse crumbs, about 10 seconds. Then, add the egg yolk and pulse again until mixed. If the mixture feels too dry, gradually add cold water, pulsing until the dough comes together into a ball.

Chill the Dough: Turn the dough out onto a lightly floured surface and knead briefly until smooth. Shape into a disk and wrap tightly in plastic wrap. Chill in the refrigerator for at least 30 minutes.

Preheat the Oven: After chilling, preheat your oven to 350°F (175°C). Roll out the chilled dough to about 1/8 inch thick. Cut out circles that will fit your tartlet molds.

Shape and Bake the Shells: Press the dough circles into the tartlet molds, prick the bottoms with a fork, and bake for 12-15 minutes or until edges are golden brown. Let cool completely.

Make the Lemon Curd: Whisk together fresh lemon juice, zest, granulated sugar, and large eggs in a heatproof bowl. Set over simmering water and cook, whisking constantly for 8-10 minutes until thickened.

Add Butter to the Curd: Once thickened, whisk in the small pieces of cold unsalted butter until smooth and creamy.

Chill the Lemon Curd: Cover the curd with plastic wrap to prevent a skin from forming and chill in the refrigerator for at least 1 hour.

Assemble the Tartlets: Spoon or pipe the lemon curd into the cooled tartlet shells generously.

Optional Garnish: Dust with powdered sugar, or add fresh berries or whipped cream.

Serve and Store: Best enjoyed fresh, can be refrigerated for up to 3 days.