Preheat your oven to 375 degrees Fahrenheit. Line a 9" pie pan with pie crust, crimp the edges, then line it with parchment paper and add pie weights. Bake the pie crust for 25 minutes. Remove the pie weights and parchment paper, and continue to bake the pie until it is lightly golden and fully cooked, about 10 minutes more. When the pie crust comes out of the oven, turn the temperature up to 400 degrees Fahrenheit.
1 pie crust
Mix the plums, sugar, vanilla, and salt in a large baking dish. Roast until the plums are tender but not falling apart, 25-30 minutes. Remove from oven and set aside to cool.
3 lb firm ripe black plums, 1 ¼ cups sugar, 1 tablespoon vanilla, ¼ teaspoon sea salt
While the plums roast, make the mascarpone. Add the mascarpone, sour cream, powdered sugar, and vanilla to a medium-sized bowl. Beat on high speed with electric beaters for 1 minute. Do not overbeat as the mascarpone may curdle.
1 cup mascarpone, ¼ cup sour cream, ¼ cup powdered sugar, 1 teaspoon vanilla
Spread the mascarpone mixture evenly over the cooked pie crust. Push it up the edges of the pie a little if you want to see it after you add the plums.
Using a slotted spoon, transfer the cooled plums from the baking dish and carefully place them on top of the mascarpone layer.
Carefully pour the juices from the plum baking dish into a medium-sized frying pan. Bring it to a boil and then simmer it until it's thickened (like balsamic glaze), about 4-5 minutes. Spoon the glaze over top of the pie and serve.
Notes
Pie crust can be baked, plums roasted, glaze reduced, and the mascarpone mixture can all be made a day ahead. Refrigerate separately and assemble before serving.