In a large bowl combine flour and salt. Using a pastry blender, cut in shortening until the mixture resembles coarse crumbs. In a liquid measuring cup, beat egg yolk lightly. Add enough milk to make 3/4 cup liquid. Stir well to combine. Stir milk mixture into flour mixture with a fork until combined (dough will be slightly sticky). Divide mixture in half.
On a well-floured surface, roll half of the dough to a 17x12-inch rectangle. Fold dough crosswise into thirds. Transfer to a 15x10x1-inch baking pan and unfold dough, pressing to fit into the bottom and sides of the pan. Sprinkle with cornflakes. Top evenly with apples. In a small bowl combine granulated sugar and cinnamon. Sprinkle mixture over apples. Roll remaining dough to a 16x11-inch rectangle. Fold dough crosswise into thirds. Place atop apples and unfold dough. Trim crust if necessary. Crimp edges or use the tines of a fork to seal. Cut slits in the top. Brush top with beaten egg white.
Bake in a 375°F oven for 50 minutes or until golden and apples are tender, covering with foil after the first 25 minutes of baking time to prevent overbrowning.
In a small bowl combine powdered sugar and 3 to 4 teaspoons milk to make a drizzling consistency. Drizzle over warm bars. Let cool completely on a wire rack. Cut into bars. Store leftover bars in refrigerator for up to 3 days.