Pour 2 cups (480ml) of whole milk into a medium saucepan over medium heat and warm until small bubbles form around the edges (scalding). Remove from heat as soon as it begins to steam — do not let it fully boil.
In a large mixing bowl, whisk together 4 egg yolks and ½ cup (100g) granulated sugar until the mixture turns pale yellow and slightly thick, about 1–2 minutes. Whisk in 3 tablespoons cornstarch until fully combined with no lumps.
Slowly ladle the warm milk into the egg mixture while whisking constantly — add a ladle at a time to gradually raise the temperature without scrambling the eggs. Then pour the tempered mixture back into the saucepan.
Set the saucepan over medium heat and whisk constantly for 3–5 minutes until the custard suddenly thickens and becomes glossy. Continue cooking and whisking for 1 more minute, then remove from heat and stir in 2 tablespoons butter and 1 teaspoon vanilla extract.
Strain the pastry cream through a fine mesh sieve into a clean bowl. Press plastic wrap directly onto the surface to prevent a skin, then refrigerate for at least 1 hour until fully set.
In a food processor, pulse flour, powdered sugar, and salt together. Add cold cubed butter and pulse 10–12 times until the mixture resembles coarse, pea-sized crumbs. Add the egg yolk and pulse, then add ice water one tablespoon at a time until the dough just comes together.
Turn the dough onto a lightly floured surface and gently shape into a flat disc without kneading. Wrap in plastic wrap and refrigerate for at least 30 minutes.
On a lightly floured surface, roll the chilled dough to a 12-inch circle, about ⅛ inch (3mm) thick. Roll it loosely around the rolling pin and unroll it over the tart pan, pressing it gently into the bottom and fluted sides. Trim excess dough, dock the bottom with a fork, then freeze for 15 minutes.
Preheat oven to 375°F (190°C). Line the frozen shell with parchment and fill with pie weights, then bake for 15 minutes. Remove weights and parchment, then bake 8–10 minutes more until light golden and dry. Cool completely on a wire rack.
Wash, dry, and prep all fruit: hull and slice strawberries, peel and slice kiwi into rounds, separate mandarin segments removing pith, and leave blueberries and raspberries whole. Pat everything thoroughly dry with paper towels.
Remove the cooled shell from the pan and place on a serving board. Stir the chilled pastry cream to loosen, then spread it evenly into the shell using an offset spatula. Arrange the fresh fruit on top, starting with larger pieces and filling in gaps with smaller ones.
Melt 2 tablespoons apricot jam with 1 tablespoon water, strain if needed, and brush warm glaze gently over all the fruit using light dabbing strokes. Refrigerate for 30 minutes to set, then serve chilled or at cool room temperature.