In a large mixing bowl, combine the all-purpose flour, cold cubed unsalted butter, granulated sugar, and salt. Using a pastry cutter or your fingertips, mix until mixture resembles coarse crumbs, about 5 minutes. Gently stir in the large egg yolk, and add cold water a tablespoon at a time if needed to form cohesive dough.

Transfer the dough to a floured surface and roll it out to about ¼ inch thick. Carefully fit the rolled dough into tartlet pans, pressing it into the sides, and trim any excess. Cover with plastic wrap and chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Remove the chilled tartlet shells and bake for 15-20 minutes until golden brown and firm to the touch. Allow the shells to cool completely on a wire rack.

In a medium saucepan, whisk together the fresh lemon juice, lemon zest, granulated sugar, and large eggs. Place over medium heat, stirring constantly until the mixture thickens, about 10 minutes.

Once thickened, remove the lemon curd from heat and stir in small pieces of unsalted butter. Mix well until fully melted and incorporated.

Pour the warm lemon curd into the cooled tartlet shells, filling them about ¾ full. Allow to set at room temperature for at least 1 hour or chill in the refrigerator for a firmer treat.

Dust with powdered sugar before serving; optionally top with fresh berries or a drizzle of chocolate.