Place rack in center of the oven then preheat to 350 degrees. Coat a 13" x 9" cake pan with cooking spray or line with parchment paper then set aside.
In a medium-sized mixing bowl, toss blackberries with sugar, lemon juice and lemon zest until well combined then set aside.
In a separate large mixing bowl, sift together flour, baking powder and salt. Cut the cold butter into flour mixture using a pastry cutter or fork until the mixture resembles coarse crumbs, then add ¾ cup granulated sugar and stir until combined.
In a small bowl, whisk together the beaten egg and vanilla then add to dry ingredients. Mix until well combined—the mixture will be dry and shaggy-looking.
Transfer 3 cups of the pastry mixture to the bottom of the prepared pan then press into an even layer. Pour blackberry filling—including juices—over the crust then spread into an even layer.
Crumble remaining pastry mixture evenly over the top of blackberries then bake 40–45 minutes until the top becomes a light golden brown. Remove from oven then allow to cool until crust becomes firm. Serve slightly warm, or at room temperature. Enjoy!
One Last Thing
Four cups of blackberries is about three 8 oz containers.