Preheat oven to 425 degrees F. Gently toss blackberries with sugar, cornstarch, and salt in a large bowl. On a floured surface, roll 1 disk of Pate Brisee to 1/8-inch thickness. Using a 5 1/2-inch biscuit cutter, cut out 6 rounds. Drape each round over a 4-inch fluted tart pan with a removable bottom, and press dough into bottom and up sides of pan. Edges will be flush with rims. Roll remaining disk of dough to 1/8-inch thickness. Cut out 6 more 5 1/2-inch rounds. Fill each tart pan with 3/4 cup blackberry mixture, mounding berries in the center. Dot berries with butter. Center 1 of the remaining dough rounds on top of 1 tart pan. Gently press top and bottom edges of dough together to seal, and then pinch sealed edges. Repeat with remaining rounds. Transfer pies to a rimmed baking sheet, and freeze or refrigerate until firm, 15 to 20 minutes. Lightly whisk yolk and heavy cream in a small bowl. Brush the top of each pie with egg wash, and sprinkle with sugar. Cut 4 vent holes in the top of each with a paring knife. Bake for 30 minutes. Rotate baking sheet, and reduce heat to 375 degrees. Bake until berries are bubbling and crust is deep golden brown, about 30 minutes. Let pies cool in pans on a wire rack for 30 minutes. Unmold, and let pies cool completely on wire rack. Serve with whipped cream.