For the tart dough:
1 egg yolk
2 tbsp. very cold water
1 tsp. vanilla extract
1 1/4 cups all-purpose flour
1/3 cup sugar
1/4 tsp. salt
8 tbsp. cold unsalted butter, cut into 1/4-inch cubes
1 Tbs. all-purpose flour
1 Tbs. cornstarch
2 egg yolks
2/3 cup milk
1/2 tsp. vanilla extract
To make the tart dough, in a small bowl stir together the egg yolk, water and vanilla; set aside. Combine the flour, sugar and salt in the bowl of a stand mixer fitted with the paddle attachment. Add the butter and beat on medium-low speed until the mixture resembles coarse cornmeal, with butter pieces no larger than small peas. Add the egg mixture and beat on low speed just until the dough comes together. Wrap in plastic wrap and refrigerate until well chilled, about 30 minutes.
Preheat the oven to 425 ° F. When you are ready to roll out the dough, unwrap it and place it on a lightly floured work surface. Roll out with a floured rolling pin, lifting and turning the dough occasionally to prevent it from sticking to the work surface. Roll it out evenly, about 1/8-inch thick. Carefully transfer the dough to the tartlette pans (I use this one from Williams and Sonoma), pressing it into the sides, and trim to remove the excess. Reroll any excess dough to line the remaining pans.
Place the tartlette pans on a baking sheet. Cover the dough with foil. Bake for 15 minutes. Reduce the oven temperature to 400° F. Remove the foil and return to the oven to bake until the shells are golden and completely baked, 5-10 minutes more. Transfer to a wire rack and let cool completely.
To make the filling, in the bowl of an electric mixer fitted with the whisk attachment, beat the granulated sugar, flour, cornstarch and egg yolks on medium speed until thick and pale yellow, 2 to 3 minutes.
In a saucepan over medium heat, combine the milk and vanilla and bring to a simmer. While stirring, pour half the milk into the sugar mixture until combined. Then pour this mixture back into the pan with the remaining milk and place over medium-low heat. Cook, whisking constantly, until the mixture thickens and starts to bubble, 1 to 2 minutes. Be sure to scrape the sides and corners of the pan while whisking to prevent scorching. Pour the pastry cream into a clean bowl and cover. Let cool to room temperature, then refrigerate for 1 hour.
Spoon the filling onto the tartlette crust and top with fruit of your choice (I used blackberries and sliced strawberries).