Preheat the oven to 375 degrees F. In the canister of a food processor, put the flour, 2 tablespoons of sugar, and salt. Pulse to combine.
Add the very cold butter to the flour mixture. Pulse only until the butter is cut into the flour.
One tablespoon at a time, add the ice water to flour/butter mixture pulsing the food processor on and off in long pulses. You’ll need somewhere between 3-6 tablespoons. I used 5 tablespoons. Stop adding the water when the flour looks evenly moistened and can stick together.
Working quickly, roll out your dough on a well floured surface, rotating frequently to prevent sticking, until about 1/8 of an inch thick. Very carefully transfer the crust to a buttered baking sheet. (I transfer my crusts by gently rolling them onto a rolling pin and gently rolling them off into whatever baking surface I’m using)
In a medium bowl, gently mixture together your blackberries, lemon zest, 1/4 cup sugar, and 2 tablespoons of cornstarch. You want the blackberries intact, but evenly covered with the other ingredients.
Pour the blackberry mixture into the center of the crust leaving 2 to 3 inches of crust all the way around the blackberries.
Working clockwise, begin folding the extra crust over the blackberries so that the blackberries are gently encased and the crust is slightly overlapping itself. Optional: brush egg white onto the crust and sprinkle with a little sugar
Place the baking sheet in the center rack of the oven and bake until the crust is barely golden brown, approximately 30-36 minutes.