Martha showcases ripe summer blueberries in an intricate tart inspired by an American fashion icon: the bandanna.
Pate Brisee: Pulse flour, granulated sugar, and salt in a food processor to combine. Pulse in butter until mixture resembles coarse meal. With machine running, add water in a slow, steady stream until dough just holds together. Divide dough in half and shape each half into a square; wrap in plastic. Refrigerate at least 1 hour and up to 2 days.
Filling: Using a potato masher, mash 1 cup blueberries with granulated sugar and cornstarch in a large bowl. Stir in remaining 7 cups blueberries until well combined. Let stand 1 hour.
Preheat oven to 450 degrees. On a lightly floured surface, roll out one dough square to an 11-inch square. Fit into a 9-inch square fluted tart pan with removable bottom, gently pressing dough into sides. Spread blueberry mixture over dough.
Roll out remaining dough square into an 11-inch square. Using small cutters, cut out shapes as desired, leaving at least a 2-inch border. If dough becomes too soft, refrigerate until firm. Carefully place dough over filling and trim overhang; press edges to seal. Refrigerate 30 minutes.
Whisk together egg and 1 tablespoon water; brush top crust with egg wash. Liberally sprinkle with sanding sugar. Place tart pan on a parchment-lined baking sheet.
Bake 25 minutes. Reduce oven temperature to 400 degrees. Bake until filling is bubbling, 30 to 35 minutes more (if crust is browning too quickly, cover with parchment-lined foil). Transfer pan to a wire rack; let cool completely. Unmold; cut into squares and serve.