For the crusts:
2 1/4 cups all-purpose flour, plus more for dusting
3/4 cup pecans
6 tablespoons granulated sugar
1/4 teaspoon salt
1 1/2 sticks cold unsalted butter, cut into 1/2-inch pieces
1 large egg, beaten
For the pastry cream:
1 cup whole milk
2/3 cup granulated sugar
1 teaspoon vanilla extract
1/2 cup plus 3 tablespoons buttermilk
3 large egg yolks
3 tablespoons cornstarch
1 pint raspberries
1 pint blueberries
Confectioners' sugar, for dusting
Make the crusts: Pulse the flour, pecans, granulated sugar and salt in a food processor until the nuts are finely ground. Add the butter and pulse until the mixture looks like coarse meal. Drizzle in 6 tablespoons ice water and pulse until the dough comes together. Turn out onto a piece of plastic wrap and form into a disk; wrap and refrigerate until firm, about 1 hour.
Lightly flour a large sheet of parchment paper. Roll out the dough on the parchment into a 9-by-13-inch rectangle. Trim into an 8-by-12-inch rectangle (reserve the scraps). Cut into eight 3-by-4-inch rectangles. Separate the pieces slightly on the parchment, then use your fingers to crimp the edges. Slide the parchment onto a baking sheet. Roll out the scraps until 1/4 inch thick. Cut out 8 stars using a small floured star-shaped cookie cutter; arrange on the baking sheet around the crusts. Refrigerate until firm, about 20 minutes.
Preheat the oven to 375 degrees F. Poke holes in the crusts with a fork. Brush the stars and the edges of the crusts with the beaten egg. Bake until golden brown, about 25 minutes; remove to a rack to cool completely.
Make the pastry cream: Combine the milk, 1/3 cup granulated sugar and the vanilla in a saucepan and bring to a simmer over medium heat. Whisk 1/2 cup buttermilk, the egg yolks, cornstarch and the remaining 1/3 cup sugar in a bowl. Slowly pour the hot milk mixture into the egg mixture, whisking constantly. Pour the mixture back into the saucepan and cook, whisking, until thick and bubbling, about 3 minutes. Transfer to a bowl; stir in the remaining 3 tablespoons buttermilk. Lay plastic wrap on the surface and refrigerate 45 minutes.
Carefully transfer the crusts to a platter. Spread the pastry cream on the crusts, then top with the berries. Top each tart with a star. Dust with confectioners' sugar.