Creme fraiche adds rich texture and tangy flavor to the filling of this streusel-topped plum pie. It also balances out the sugar in the fruit.
Pate Sucree: Pulse flour, granulated sugar, and salt in a food processor until combined. Pulse in butter until mixture resembles coarse meal, about 10 seconds. Pulse in egg yolk. With machine running, add ice water in a slow, steady stream just until dough holds together. Turn dough out onto a work surface; shape into a disk and wrap in plastic. Refrigerate at least 1 hour and up to 2 days.
Streusel: Whisk together confectioners' sugar, flour, baking powder, and salt in a medium bowl. Using a pastry blender or your fingers, cut in butter until mixture resembles coarse crumbs.
On a lightly floured surface, roll out dough about 1/8 inch thick. Fit into a 9 1/2- or 10-inch pie dish (about 1 1/2 inches deep). Using kitchen shears or a paring knife, trim to a 1-inch overhang; fold under. Crimp edges as desired. Prick bottom of dough all over with a fork. Freeze 30 minutes.
Preheat oven to 400 degrees with rack in lower third. Line dough with parchment, pressing it into corners and edges; fill with dried beans or rice. Bake, covered with foil, 20 minutes. Remove beans or rice and parchment (keep edges covered with foil). Bake until pale golden brown, 12 to 16 minutes more. Transfer to a wire rack; let cool slightly. Reduce oven temperature to 375 degrees.
Filling: In a medium bowl, gently toss together plums, granulated sugar, and salt; let stand 15 minutes. Spread 2 tablespoons creme fraiche onto bottom of crust; sprinkle with one-third of streusel. Arrange plums on top; spread or dot with remaining 3 tablespoons creme fraiche. Sprinkle with remaining streusel.
Bake until creme fraiche is bubbling and streusel is golden brown, about 50 minutes (if edges of crust are browning too quickly, tent with foil). Transfer to a wire rack and let cool 15 minutes. Serve warm or at room temperature with ice cream or whipped cream, if desired.