In a large bowl, combine flour, sugar and baking powder. Cut in shortening and butter until mixture resembles coarse crumbs. Gradually add milk, tossing with a fork until dough forms a ball. Divide dough into three portions. Refrigerate until chilled., Meanwhile, for filling, in a large bowl, combine Zwieback crumbs, salt and pepper. Crumble beef and pork over mixture and mix well. Shape rounded tablespoonfuls of meat mixture into 3-in. logs; set aside., Preheat oven to 350°. On a floured surface, knead one portion of dough 8-10 times. Roll dough to 1/8-in. thickness; cut with a floured 3-in. round cutter. , Place one log in the center of each circle. Brush edges of dough with water; fold dough over filling and pinch edges to seal. Reroll scraps. Repeat with remaining dough and filling., Place on greased racks in shallow baking pans. Bake 35-40 minutes or until a thermometer reads 160°. Serve with mustard if desired.