Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a mixing bowl, combine flour, sugar, and salt. Add cold butter and mix until the mixture resembles coarse crumbs.
Add ice water, one tablespoon at a time, until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.
On a floured surface, roll out the dough into a 12-inch circle. Transfer to the prepared baking sheet.
In a bowl, toss blackberries with sugar and cornstarch. Arrange in the center of the dough, leaving a 2-inch border.
Fold the edges of the dough over the blackberries, pleating as needed. Brush the edges with beaten egg.
Bake for 45 minutes or until the crust is golden and the filling is bubbly. Let cool before serving.