Combine flour and salt; cut in shortening. Beat egg yolk in measuring cup; add enough milk to make 2/3 cup liquid. Mix well; stir into flour mixture.
On floured surface, roll half of the dough into a 17x12-inch rectangle; fit into and up sides of 15.5x10.5x1-inch baking pan.
Sprinkle with corn flakes and top with apples. Combine sugar and cinnamon; sprinkle on top. Roll remaining dough to 15.5x10.5-inch rectangle. Place over apples.
Seal edges; cut 5 slits in the top. Beat egg white until frothy; brush on crust. Bake in 375 degree oven for 50 minutes. Prepare powdered sugar glaze; drizzle on warm cookies. Makes 3 dozen.