In a bowl, toss 6 cups of blackberries, 1 cup of sugar, 3 tablespoons of cornstarch, 1 teaspoon of lemon juice and 1/2 teaspoon of grated lemon zest. Let it stand for 20 minutes while mixing and mashing it occasionally. Fit one disk of pie dough into a 9-inch pie plate, and trim the edges to leave 1/2-inch of overhang. Refrigerate it, and cut another disk of dough into 11 strips 11/4-inch wide. Spoon the blackberry mixture into the pie shell, and dot the top with 2 tablespoons of butter. Place the dough strips in a lattice pattern on top of the filling, trim them so that they're close to the edge of the pan, and roll up and crimp the edge of the pie shell.
Lightly beat one large egg with 1 tablespoon of water, and brush it over the top and edges of the pie. Put the pie on a baking sheet in an oven preheated to 400 degrees Fahrenheit, and bake it until the filling starts bubbling. Allow the pie to cool before serving it.Learn more about Desserts