01 - Preheat your oven to 350°F. Line a baking sheet with parchment paper, ensuring it covers the entire surface.
02 - In a large mixing bowl, combine the all-purpose flour, powdered sugar, and salt. Add the cold, cubed unsalted butter and cut it into the dry ingredients using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Incorporate the large egg yolk and 2 tablespoons of cold water, mixing until the dough just comes together. If the dough is too dry, add more cold water, one tablespoon at a time, until a cohesive dough forms. Shape the dough into a rectangle, wrap it tightly in plastic film, and refrigerate for 20 minutes.
03 - On a lightly floured surface, roll out the chilled pastry dough to a uniform 1/4-inch thickness, forming a rectangle approximately 9x12 inches. Carefully trim the edges of the dough for a neat and clean appearance. Transfer the pastry to the prepared baking sheet. Prick the entire surface of the dough with a fork to prevent it from puffing excessively during baking. Bake for 20–25 minutes, or until the pastry crust is golden brown. Allow the baked crust to cool completely on the baking sheet.
04 - In a medium saucepan, gently heat the whole milk with the vanilla bean (if using) over medium heat until it reaches a gentle simmer. In a separate bowl, whisk together the large egg yolks, granulated sugar, and cornstarch until the mixture is smooth and pale. Gradually temper the hot milk into the egg yolk mixture, continuously whisking to prevent the eggs from scrambling. Return the entire mixture to the saucepan and cook over medium heat, whisking constantly, for 3–4 minutes, or until the cream has thickened considerably. Remove the saucepan from the heat, discard the vanilla bean (if used), and stir in the unsalted butter until fully melted and incorporated. Allow the vanilla cream to cool completely before spreading it evenly over the cooled pastry base.
05 - Artistically arrange the prepared fruits—sliced strawberries, raspberries, blueberries, sliced kiwifruit, and optional red currants or pomegranate seeds—on top of the cooled vanilla cream. Arrange them in a design that resembles an envelope or a 'love letter,' as desired.
06 - In a small saucepan, gently heat the apricot jam with 1 tablespoon of water over low heat until the jam is melted and smooth. Strain the glaze through a fine-mesh sieve to remove any fruit solids, ensuring a clear finish. Using a pastry brush, carefully apply a thin, glossy layer of the warm glaze over the arranged fruits to enhance their appearance and prevent them from drying out.
07 - Place the finished fruit tart in the refrigerator and chill for a minimum of 30 minutes before slicing into individual portions and serving. This allows the tart to set properly.