Chocolate Crust:1 cup all-purpose flour1/4 cup sugar1/4 Teaspoon salt6 tablespoons cold unsalted butter, cut into 12 pieces3 tablespoons unsweetened cocoa, preferably Dutch-process1/2 teaspoon vanilla extract2 to 3 tablespoons ice waterFilling:4 ounces unsweetened chocolate1 stick (4 ounces) unsalted butter3 eggs1 cup sugar1/4 teaspoon salt1 teaspoon vanilla extract2/3 cup all-purpose flour1/2