Old-Fashioned Blackberry Cobbler

Name: Old-Fashioned Blackberry Cobbler
Prep time: 30 min
Cook time: 1 h
Total time: 1 h 30 min
Cuisine: American
Ingredients: 1 double pie crust recipe; 1 cup granulated sugar; 1 to 2 tablespoons cornstarch ((see notes)); ⅛ teaspoon ground cinnamon; 1 cup boiling water; 4 cups blackberries; juice of ½ lemon ((if needed to add tartness)); 1 tablespoon cold unsalted butter (cut into small cubes); 1 large egg (lightly beaten); coarse sugar for topping