Add flour, salt and sugar into a large bowl. Mix well evenly. Add cold butter cubes and cut them into the flour until crumbly texture forms nicely.

Pour ice water slowly into the dough mixture. Stir gently until the dough comes together. Divide into two equal portions and shape each portion into flat discs.

Wrap both dough discs with plastic wrap carefully. Place them inside the refrigerator for at least thirty minutes. Chilling helps create a flaky and buttery pie crust.

Add blackberries, sugar, cornstarch, lemon juice, vanilla extract and cinnamon into a bowl. Stir carefully until the berries become evenly coated with the sweet pie filling mixture.

Lightly flour your work surface. Roll one dough disc into a large circle carefully. Place it gently inside the pie dish and press lightly into place.

Pour the blackberry filling evenly into the prepared pie crust. Spread the berries gently with a spoon so the filling cooks evenly while baking inside the oven.

Roll the second dough disc carefully. Place it over the blackberry filling. Seal the edges gently and cut small slits on top for steam to escape properly.

Brush beaten egg evenly across the top crust using a pastry brush. Sprinkle sugar lightly on top for extra sweetness, shine, and beautiful golden color after baking completely.

Bake the pie at 375°F for about fifty minutes. The crust should become golden brown and the blackberry filling should bubble around the edges while baking perfectly.

Remove the pie from the oven carefully. Let it cool on a rack for at least one hour. This helps the blackberry filling set properly before slicing.