For crust: 1 lb. of all-purpose flour 2 Tbsp. of sugar 1 Tbsp. of salt 2 sticks cold butter, cubed 2 egg yolks 3⁄ cup of ice water 4 For filling: 8 cups of fresh blackberries 3⁄ cup of sugar 4 1⁄2 cup of cornstarch 4 tsp. of vanilla extract For egg wash: 1 egg 1 Tbsp. water 2 Tbsp. turbinado sugar (raw sugar)
Preheat oven to 350 degrees. In a medium bowl, sift flour, sugar and salt together. Using an electric mixer with a paddle attachment, break up cold butter in the flour mixture until the size of peas, then add egg yolks. Quickly add ice water and blend slowly until the dough just comes together. On a floured surface, knead the dough together into a ball and pat down to a flat disc. Wrap in plastic wrap and refrigerate for half an hour before rolling.
In a bowl, gently mix together the berries, sugar, cornstarch and vanilla.
Divide the dough in half and roll into two rectangular pieces, one 14 inches by 17 inches and another 10 inches by 14 inches. Place the larger rectangle in the bottom of a 9-inch-by-13-inch baking dish. Pour in blackberry mixture. Cover with the smaller piece of dough. Gently crimp together the dough all around to seal the edges.
For egg wash, beat egg with water in a small bowl. Using a pastry brush, brush entire top layer of dough with egg wash and sprinkle turbinado sugar on top. Bake for 50 to 60 minutes. After 30 minutes, cover with foil and continue baking.