This elegant dessert is almost like a French clafoutis, with lush ripe fruit nestled in a creamy egg filling. Buttermilk gives it a tang that makes the filling almost like cream cheese, and the lemon keeps it bright and fresh tasting. The ripe raspberries seep a little of their purple-pink juice into the white filling, and the whole thing is so pretty you’ll hardly want to cut into it. This is a tart, wide and shallow, rather than a deep pie, so be sure that you form the piecrust into a 10-inch tart pan with a removable bottom.
Stir together the flour, sugar, and salt in a medium bowl.
Cut the butter into 8 or 10 pieces and use a fork, a pastry cutter, or two knives to cut the butter into the flour until the mixture resembles coarse crumbs.
Stir in the buttermilk all at once. Gather the pastry into a ball and wrap it in plastic wrap, pressing it down into a flat disk. Refrigerate for 1 hour, or until firm. (At this point, you can refrigerate for up to 3 days, or store in the freezer for 3 weeks.)
When ready to use, roll out the dough on a floured surface. Ideally roll forward with a wide stroke, then turn and roll again, repeating until the dough is evenly rolled. This way, you don't roll back and forth and possibly misshape your pie crust. You may need to sprinkle the work surface and the dough with a little more flour as you work, so it will turn readily.
Arrange the rolled dough over the rolling pin and transfer it to a 10-inch fluted tart pan.
Preheat the oven to 350 degrees. Set the tart pan on a baking sheet. Bake the piecrust for 15 to 20 minutes, until lightly browned. Let the crust cool slightly, and then spread the raspberries in the crust.
In a medium bowl, whisk the sugar with the flour. Mix in the egg yolks and lemon juice and zest, and then beat in the butter. Whisk in the buttermilk and vanilla until smooth. Pour the mixture over the raspberries.
Set the baking sheet in the oven and bake for 25 to 30 minutes, until the filling is just set in the center. Let cool before serving.