Prepare a double crust pie pastry and refrigerate until needed. (See link in notes)
Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper, set aside. Lightly grease an 8 or 9-inch casserole dish with vegetable cooking spray. Set aside.
In a medium saucepan mix together 1 cup of sugar, cornstarch, cinnamon and boiling water. Bring the sugar mixture to a boil over medium heat. Boil 5 minutes stirring occasionally until thickened. Remove from the heat and add the blackberries and any accumulated juices. Stir gently until the blackberries release some juice. Set aside.
Cut one of the pastry discs in half and roll out on a lightly floured to about 1/8th inch thick. Cut the pastry into a square roughly the same size as the prepared casserole dish. Reserve any pastry scraps. Place the pastry square on the parchment lined baking sheet and bake until firm, lightly browned with bubbles starting to form on the crust, about 10 minutes. This will be used for the middle layer of the cobbler. Set aside.
While the pastry square is baking, roll out the remaining whole disc of dough to at least a 12x12-inch square, about 1/8th inch thick. Gently ease the dough into the prepared dish taking care to press it into the corners without stretching the dough. Trim the edges of the dough leaving a 1/2-inch overhang all the way around. This overhang will be folded up and over the lattice top. Collect any scraps of dough and put them together with the remaining 1/2 disc of pastry.
Spoon half the blackberry mixture into the prepared pan. Top with the pre-baked pastry square, then all the remaining blackberry mixture. Dot the blackberries with butter.
Roll out the remaining dough to a 10x10-inch square. Cut the dough into 8 or 10 strips and weave a lattice crust on top. Trim the excess dough to the same size as the overhang. Lightly dampen the top and bottom edges of the dough strips with a wet finger. Fold the excess overhang up and over the edges of the strips to seal. Crimp or seal the edges with a fork.
Brush the pastry with the beaten egg and sprinkle with coarse sugar.
Place the casserole dish on the parchment lined baking sheet to catch any juice that may bubble over. Bake at 400°F for 10 minutes, then reduce the heat to 350°F and continue baking for about 45 to 55 minutes or until the crust is golden brown and the juices are bubbling. If the crust is browning too quickly, tent the cobbler with foil as needed.
Cool to room temperature or serve warm topped with ice cream or lightly sweeten cream if desired.