In a mixing bowl, combine flour, sugar, and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

In another bowl, combine blackberries, sugar, cornstarch, lemon juice, and vanilla extract. Toss gently to coat the berries.

Preheat the oven to 375?F (190?C). Roll out the chilled dough on a floured surface into a rough circle about 12 inches in diameter.

Transfer the dough to a parchment-lined baking sheet. Spoon the blackberry mixture into the center, leaving a 2-inch border.

Fold the edges of the dough over the filling, pleating as necessary. Brush the crust with beaten egg and sprinkle with sugar.

Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown and the filling is bubbly.

Allow to cool slightly before slicing and serving.