In a food processor, combine all-purpose flour, granulated sugar, and a pinch of salt. Pulse in cold, cubed butter until the mixture resembles coarse crumbs, about 30 seconds. Add the egg yolk and pulse to combine, adding cold water gradually if the mixture seems too dry. Once a dough forms, knead briefly, wrap it in plastic wrap, and chill for 30 minutes in the refrigerator.

Preheat your oven to 350°F (175°C).

After chilling the dough, roll it out on a lightly floured surface to about 1/8 inch thick. Using a cookie cutter, cut out circles that fit your tartlet molds, then press the dough gently into the molds, ensuring an even thickness. Prick the bottoms with a fork to prevent bubbles during baking.

Bake the tartlet shells in your preheated oven for 12-15 minutes, or until golden brown. Let them cool completely on a wire rack.

While the tartlet shells cool, whisk together fresh lemon juice, grated lemon zest, granulated sugar, and large eggs in a heatproof bowl over simmering water. Continue whisking constantly for about 8-10 minutes, or until the mixture thickens and coats the back of a spoon.

Remove from heat and stir in cold butter pieces until fully melted and smooth. For a silky texture, strain the curd through a sieve.

Once your lemon curd is ready, cover it with plastic wrap, ensuring it touches the surface to prevent a skin from forming. Refrigerate the curd for at least 1 hour to allow it to set.

After chilling, fill each cooled tartlet shell generously with the chilled lemon curd until they are just full. Smooth the tops with a spatula for an elegant finish. Optionally, add toppings like powdered sugar or fresh berries.

Serve your delightful lemon tartlets fresh for the best flavor and texture. They can be stored in the refrigerator for up to 3 days in an airtight container.