To prepare filling, combine first 5 ingredients; set aside.
To prepare pastry, lightly spoon flour into dry measuring cups; level with a knife. Combine 1/2 cup flour, ice water, and vinegar, stirring with a whisk until well blended to form a slurry. Combine 1 1/2 cups flour, powdered sugar, and salt in a bowl; cut in shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Add slurry; toss with a fork until moist. Gently press mixture into a 4-inch circle.
Place dough on 2 sheets of slightly overlapping plastic wrap on a slightly damp surface. Cover dough with 2 additional sheets of plastic wrap. Roll dough, still covered, into a 16 x 12-inch rectangle. Slide dough onto a baking sheet; place in freezer 10 minutes or until the plastic wrap can be easily removed. (Keep chilled until ready to use.)
Preheat oven to 400°.
Remove top sheets of plastic wrap. Place dough, plastic wrap side up, on a flat surface. Remove remaining sheets of plastic wrap. Cut pastry into 12 (4-inch) squares. Place about 1 tablespoon filling into center of each square. Moisten the edges of squares with 1 tablespoon milk; bring 2 opposite corners together. Press edges together with a fork to seal, forming a triangle. Place triangles on a large baking sheet. Brush tops with 1 tablespoon milk; sprinkle evenly with granulated sugar. Bake at 400° for 18 minutes or until golden. Serve warm or at room temperature.