In a food processor, combine the flour and the salt. Add the butter and pulse for a few seconds at a time until it forms small, pea sized pieces. Add the cold water and pulse again until dough forms. 

Remove the dough from the food processor and roll into a cylinder. Cover with plastic wrap and refrigerate for 30 minutes. 

Preheat the oven to 425°F. 

Line a baking sheet with parchment paper. 

Divide the chillded dough into six equal sections. 

Working on a lightly floured surface, use your hands to make each section into a disc shape. Then using a rolling pin, roll each disc into a 6" circle that is about 1/8" thick. Set the dough on the lined baking sheet and set aside. 

In a small bowl, combine the cream and the egg yolk. Set aside. 

In a second bowl, combine the sugar and the cornstarch. Add the blackberries and mix gently. 

Divide the blackberry mixture evenly among the dough, spooning it onto the center of the dough and folding the edges of the dough toward the center. You want to cover the outside of the filling with the dough but leave the center uncovered. 

Brush the dough with the egg mixture in the small bowl and sprinkle with sugar, if desired. 

Bake for 20 minutes or until the crust is golden brown. 

Cool before serving.