In large mixing bowl, stir together flour and salt. Cut in butter with pastry blender until mixture is crumbly.
In 1 or 2-cup measure, beat egg yolk lightly with fork, adding just enough milk to make 2/3 cup. Stir milk and egg yolk mixture into flour mixture, blending with fork just until mixture holds together and comes away from side of bowl. Divide dough into 2 portions. Roll half the dough out to line a 10 1/2- x 15 1/2- x 1-inch baking pan (jelly roll pan). Place dough in pan, pressing lightly into bottom and up sides to form the bottom crust. Sprinkle bottom crust with crushed cornflakes. In large bowl combine apple slices with sugar, ground ginger, and ground cinnamon; stir to blend well. Arrange apple mixture over bottom crust. Roll out remaining dough and place over top of apples. Pinch edges of dough together to seal. Brush top crust with lightly beaten egg white. Bake at 400° for 50 to 60 minutes, or until golden brown. Combine confectioners? sugar and vanilla; stir in enough water to make frosting. Let pastry cool slightly then spread (or drizzle) with frosting while still warm. Cut into squares; makes about 16.