Line a large cookie sheet with parchment paper.
In a large bowl whisk together the flour, sugar, baking powder, and salt.
Add the cold diced butter and using your fingertips, rub the butter into the flour mixture until it resembles coarse crumbs.
In a separate small bowl whisk with a fork the heavy cream, buttermilk and vanilla until combined.
Create a small well in the center of the flour mixture. Pour the cream mixture into the well and stir with a fork until the dough comes together. Toss in the prunes and pistachios and mix gently to incorporate.
Form the scones: Using a large ice cream scoop or spoon, dollop the scone dough in 1/3-cupfuls onto a prepared baking sheet. You can also roll dough into a circle, about 3/4 of an inch thick and use a round cookie or biscuit cutter to cut out about 12 scones, reshaping the dough as needed and using scraps to form new rounds of dough.
For best texture and highest rise, place the pan of scones in the freezer for 15-30 minutes, uncovered.
While the scones are chilling, preheat the oven to 375º F(190º C).
Brush the scones with heavy cream and bake for about 18 to 20 minutes or until risen and golden brown. Remove from oven and allow to cool down completely on a wire rack.
Dust with confectioner's sugar, if desired. Serve.