In a mixing bowl, whisk together the flour, sugar, and salt until well combined. Cut in the cold unsalted butter using a pastry cutter until the mixture resembles coarse breadcrumbs. In a separate bowl, whisk the egg yolk, vanilla extract, and milk; then pour into the flour mixture. Mix until a dough forms, shape it into a disc, wrap, and chill for 1 hour.

While the dough chills, preheat your oven to 375°F (190°C). Prepare your tartlet pans by greasing them lightly. Once the dough has chilled, roll it out on a floured surface to about 1/8 inch thickness. Carefully fit the rolled dough into the tart pans, pressing it into the edges. Freeze the crusts for 20-30 minutes to maintain their shape during baking.

After chilling, bake the tartlet crusts in the preheated oven for 14-17 minutes, or until golden brown and firm to the touch. Keep an eye on them to prevent over-baking; they should not darken too much. Once done, remove them from the oven and let them cool slightly while you prepare the lemon curd.

In a heatproof bowl over simmering water, whisk together the fresh lemon juice, whole eggs, egg yolks, lemon zest, sugar, and a pinch of salt. Stir continuously until the mixture thickens and reaches 175°F (79°C), about 10-12 minutes. Remove from heat and whisk in the cold unsalted butter until completely melted and smooth.

Preheat your oven again to 325°F (163°C). Carefully pour the prepared lemon curd into the baked tartlet shells, filling them just to the top. Bake for another 10 minutes, allowing the curd to set gently. After baking, let them cool in the pans for a few minutes before transferring to a wire rack.

Once cooled to room temperature, chill them in the refrigerator for at least 1 hour to firm up the curd for easier slicing. Before serving, garnish with fresh berries or a sprinkle of powdered sugar.