Make the tart shell dough: In a medium bowl, whisk together flour, powdered sugar, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse meal with pea-sized butter pieces.

Add the egg yolk and 2 tablespoons ice water. Stir with a fork until the dough just comes together. If it feels dry, add the remaining water, 1 teaspoon at a time. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

While dough chills, preheat oven to 375°F (190°C).

On a lightly floured surface, roll the dough to about 1/8-inch thickness. Cut into 6 rounds slightly larger than your tartlet pans. Gently press each round into the pans, trimming the edges flush with the rim. Prick the bottoms with a fork.

Line each shell with a small piece of parchment paper and fill with pie weights or dried beans. Place the pans on a baking sheet. Bake for 12 minutes, then remove the parchment and weights. Bake another 5-7 minutes until the shells are pale golden and dry to the touch. Set aside to cool completely.

Make the lemon curd: In a medium saucepan, whisk together eggs and granulated sugar until smooth. Whisk in lemon juice and zest.

Place the saucepan over medium-low heat, stirring constantly with a whisk. Cook for 6-8 minutes, until the mixture thickens enough to coat the back of a spoon and reaches 170°F (77°C) on an instant-read thermometer. Do not let it boil, or the curd will scramble.

Immediately remove from heat and pour the curd through a fine-mesh strainer into a clean bowl to catch any cooked egg bits. Let it cool for 2 minutes, then whisk in the butter, one piece at a time, until fully incorporated and smooth.

Pour the warm curd into the cooled tart shells, filling almost to the rim. Let the tarts cool to room temperature (about 20 minutes), then refrigerate for at least 2 hours, until the curd is fully set.

Garnish with fresh berries or a dusting of powdered sugar just before serving.