- In a food processor, combine the flour and salt. Add the butter and pulse a few seconds at a time until it forms pea-sized pieces. Add the water and pulse again until the dough just begins to form. Remove the dough from the food processor and, using your hands, roll into a cylinder. Cover with plastic wrap and refrigerate for 30 minutes.
- With the rack in the middle position, preheat the oven to 425°F (220°C). Line a baking sheet with a silicone mat or parchment paper.
- Divide the dough into six equal parts and form into discs. On a lightly floured work surface, using a rolling pin, roll each disc into a 6-inch (15 cm) circle about 1/8 inch (3 mm) thick. Set aside.
- In a small bowl, combine the cream and egg yolk. Set the egg wash aside.
- In another bowl, combine the sugar and cornstarch. Add the blackberries and mix gently until the sugar is moistened. Divide the blackberry mixture evenly among the circles of dough, placing it in the centre. Fold the edges of the dough toward the centre, over the filling but not completely covering the blackberries, forming pleats as you go. Transfer to the baking sheet. Brush the dough with the egg wash and sprinkle with sugar.
- Bake for 20 minutes or until the crust is golden. Let cool on a wire rack.