Prepare the Tart Crust:

1. Make the Tart Dough:
   In a food processor, combine the all-purpose flour, sugar, and salt. Add the cold cubed butter and pulse until the mixture resembles coarse crumbs. Add the cold water, one tablespoon at a time, and pulse until the dough starts to come together. Be careful not to overwork the dough. If needed, add more water, just enough to make the dough hold together when pressed.

2. Chill the Dough:
   Turn the dough out onto a piece of plastic wrap, shape it into a disk, and wrap it tightly. Refrigerate the dough for at least 30 minutes. This step helps relax the gluten and ensures that the crust bakes up tender and flaky.

3. Preheat the Oven:
   Preheat your oven to 350°F (175°C). Lightly grease a 12-cup muffin tin or tartlet pan. Set aside.

4. Roll out the Dough:
   Once the dough has chilled, place it on a lightly floured surface and roll it out to about ⅛ inch thickness. Cut out circles of dough large enough to fit into the muffin cups, using a round cookie cutter or a glass. Press the dough into each muffin cup and trim off any excess.

5. Blind Bake the Crusts:
   Line the tart crusts with parchment paper or aluminum foil, then fill each one with pie weights or dried beans to prevent the crust from puffing up. Bake in the preheated oven for 15-20 minutes, or until the edges are golden brown. Remove from the oven and set aside to cool.

Make the Custard Filling:

6. Prepare the Custard:
   In a medium saucepan, whisk together the milk, egg yolks, sugar, and cornstarch until smooth. Place the saucepan over medium heat and cook, whisking constantly, until the mixture thickens and begins to simmer. This should take about 5-7 minutes. Once it reaches a pudding-like consistency, remove the pan from the heat.

7. Add Butter and Vanilla:
   Stir in the butter and vanilla extract until the butter has melted and the custard is smooth. Allow the custard to cool slightly before filling the tart shells.

Assemble the Tarts:

8. Fill the Tart Shells:
   Spoon the warm custard into the cooled tart shells, filling each one to the top. Use the back of a spoon to smooth the surface.

9. Top with Raspberries:
   Gently arrange fresh raspberries on top of each custard-filled tart. You can arrange them in a circle or simply scatter them across the top.

10. Chill the Tarts:
    Place the assembled tarts in the refrigerator and allow them to chill for at least 30 minutes. This will help the custard set properly and make them easier to serve.

Serve the Tarts:

11. Serve and Garnish:
    Before serving, you can dust the tarts with powdered sugar for a little extra sweetness and a lovely touch. Serve them chilled or at room temperature, depending on your preference.