Part 1: Making the Sweet Shortcrust Pastry (Pâte Sucrée):

1. Combine Dry Ingredients: In a large bowl, whisk together the all-purpose flour, granulated sugar, and salt. This ensures the dry ingredients are evenly distributed before incorporating the butter. Even distribution is key for consistent texture and flavor throughout the crust.
2. Incorporate Cold Butter: Add the very cold, cubed butter to the flour mixture. Using your fingertips or a pastry blender, cut the butter into the flour until the mixture resembles coarse crumbs. You should still see small pieces of butter, about the size of peas. This step is crucial for creating flakiness. The goal is to keep the butter cold and prevent it from melting into the flour. Working quickly and efficiently is important. If your kitchen is warm, you might need to periodically chill the bowl in the refrigerator.
3. Add Egg Yolk and Ice Water: In a small bowl, lightly whisk the egg yolk. Add the egg yolk to the flour mixture. Gradually add ice water, one tablespoon at a time, mixing gently after each addition, until the dough just comes together. Be careful not to add too much water, as this can make the dough tough. The dough should be moist but not sticky. It should come together when you pinch a bit of it between your fingers.
4. Form Dough and Chill: Turn the dough out onto a lightly floured surface. Gently bring the dough together into a disc about 1 inch thick. Avoid overworking the dough, as this will develop the gluten and make the crust tough. Wrap the dough tightly in plastic wrap and chill in the refrigerator for at least 1 hour, or preferably 2 hours. Chilling allows the gluten to relax, making the dough easier to roll out and preventing it from shrinking during baking. It also allows the butter to firm up again, which is essential for flakiness. You can even chill the dough overnight for added convenience.
5. Roll Out Dough: Once chilled, remove the dough from the refrigerator and let it sit at room temperature for about 5-10 minutes to soften slightly. On a lightly floured surface, roll out the dough into a circle about 12 inches in diameter and about ⅛ inch thick. Rotate the dough as you roll to ensure an even circle and prevent sticking. If the dough starts to stick, dust the surface and rolling pin with a little more flour.
6. Transfer to Tart Pan: Carefully transfer the rolled-out dough to a 9-inch tart pan with a removable bottom. Gently press the dough into the pan, ensuring it fits snugly into the corners and up the sides. Trim off any excess dough hanging over the edge of the pan by running your rolling pin over the top edge or using a knife. Prick the bottom of the tart crust all over with a fork. This prevents the crust from puffing up during baking.
7. Chill Again: Place the tart pan with the dough back in the refrigerator for at least 30 minutes. This step is crucial for preventing shrinkage during baking. Chilling the dough again allows the butter to firm up completely before it goes into the oven.
8. Blind Bake Crust: Preheat oven to 375°F (190°C). Line the chilled tart crust with parchment paper and fill with pie weights, dried beans, or rice. This will weigh down the crust and prevent it from puffing up and losing its shape during baking. Bake for 20 minutes. Remove the parchment paper and pie weights. Return the crust to the oven and bake for another 15-20 minutes, or until golden brown and fully baked. The crust should be crisp and cooked through. If the edges are browning too quickly, you can cover them with foil or pie shields.
9. Cool Crust Completely: Remove the baked tart crust from the oven and let it cool completely in the tart pan on a wire rack before filling. Cooling completely ensures the crust is firm and stable enough to hold the pastry cream and berries.

Part 2: Making the Pastry Cream (Crème Pâtissière):

1. Heat Milk: In a medium saucepan, combine the whole milk and half of the granulated sugar (¼ cup). Heat over medium heat until just simmering. Stir occasionally to dissolve the sugar. Keep a close eye on the milk and do not let it boil.
2. Whisk Egg Yolks and Sugar: In a separate medium bowl, whisk together the egg yolks, remaining granulated sugar (¼ cup), cornstarch, and salt until pale and smooth. Whisk vigorously to ensure there are no lumps of cornstarch. This mixture is called a “liaison” and will thicken the pastry cream.
3. Temper Egg Yolks: Gradually whisk about ½ cup of the hot milk into the egg yolk mixture, whisking constantly. This process is called tempering and prevents the egg yolks from scrambling when added to the hot milk. Continue to whisk in another ½ cup of hot milk, still whisking constantly.
4. Combine Mixtures and Cook: Pour the tempered egg yolk mixture into the saucepan with the remaining hot milk. Reduce the heat to medium-low and cook, whisking constantly, until the pastry cream thickens significantly and comes to a simmer. This should take about 2-3 minutes. Continue to cook and whisk for another minute, ensuring the pastry cream is smooth and thickened. It should be thick enough to coat the back of a spoon and hold its shape. Be sure to whisk constantly and scrape the bottom and sides of the saucepan to prevent the pastry cream from scorching or sticking.
5. Stir in Butter and Vanilla: Remove the saucepan from the heat. Stir in the softened butter and vanilla extract until smooth and fully incorporated. The butter adds richness and shine to the pastry cream.
6. Cool Pastry Cream: Pour the pastry cream into a clean bowl. Press a piece of plastic wrap directly onto the surface of the pastry cream to prevent a skin from forming. Chill in the refrigerator for at least 2 hours, or preferably overnight, until completely cold and set. Chilling allows the pastry cream to thicken further and develop its flavor.

Part 3: Assembling the Fruit Tart:

1. Prepare Berries: Gently wash and dry all the fresh berries. Hull the strawberries if using and slice any larger berries into halves or quarters. Ensure the berries are completely dry before arranging them on the tart, as excess moisture can make the tart soggy.
2. Fill Tart Crust: Once the pastry cream is chilled and set, remove it from the refrigerator. If it has become too thick, you can whisk it briefly to loosen it slightly. Spoon the pastry cream into the cooled tart crust and spread it evenly in a smooth layer.
3. Arrange Berries: Arrange the fresh berries on top of the pastry cream in a decorative pattern. You can arrange them in concentric circles, rows, or a freeform style. Get creative and have fun with the arrangement! Consider arranging berries of different colors and sizes together for visual appeal.
4. Glaze (Optional): If desired, prepare a glaze by warming apricot jam or jelly with a tablespoon or two of water in a small saucepan until melted and smooth. Strain the glaze through a fine-mesh sieve to remove any lumps. Brush the glaze lightly over the berries to add shine and preserve their freshness.
5. Chill (Optional): For best results, chill the assembled fruit tart in the refrigerator for at least 30 minutes before serving. This allows the pastry cream to firm up further and the flavors to meld together. However, the tart is also delicious served immediately.