Combine flour, salt and 1 tbsp sugar in a bowl to make the crust.
Cut in cold butter with a pastry cutter until pea-size crumbs form.
Add ice water 1 tbsp at a time until dough just holds; form two discs and chill 1 hour.
Toss blackberries with sugar, cornstarch (or flour), lemon juice, zest and cinnamon; let macerate 10–15 minutes.
Preheat oven to 400°F (200°C). Roll out one dough disc to line a 9-inch pie pan and chill 15 minutes.
Fill crust with blackberry mixture, spreading evenly and keeping juices in the pan.
Roll second disc, cut into strips and weave a lattice over the berries; trim and crimp edges.
Brush lattice with egg wash and sprinkle a little sugar on top.
Bake at 400°F (200°C) for 20 minutes, then reduce to 350°F (175°C) and bake 25–35 more minutes until filling bubbles.
Cool on a rack at least 2 hours to set before slicing.