Step 1: Prepare the Tart Crust

1. In a large mixing bowl, whisk together the flour, powdered sugar, and salt.
2. Add the cold, cubed butter and mix with your fingertips or a pastry cutter until the mixture resembles coarse crumbs.
3. Stir in the egg yolk and add cold water, one tablespoon at a time, mixing until the dough comes together. Do not overmix.
4. Press the dough into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Step 2: Bake the Tart Shell

1. Preheat your oven to 350°F (175°C).
2. Roll out the chilled dough on a lightly floured surface to fit a 9-inch tart pan.
3. Press the dough into the pan, trimming any excess. Prick the bottom with a fork to prevent puffing.
4. Bake for 15-20 minutes or until golden brown. Allow it to cool completely.

Step 3: Make the Custard Filling

1. In a medium saucepan, combine the heavy cream, milk, and half of the sugar over medium heat. Stir until just below boiling.
2. In a separate bowl, whisk the egg yolks, cornstarch, and the remaining sugar until smooth.
3. Gradually pour the hot cream mixture into the egg yolk mixture, whisking constantly.
4. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened (about 2-3 minutes).
5. Remove from heat and stir in the vanilla extract. Let it cool slightly.

Step 4: Assemble the Tart

1. Pour the custard filling into the cooled tart shell, spreading it evenly.
2. Top the custard with a beautiful arrangement of fresh berries.
3. Chill in the refrigerator for at least 30 minutes before serving.