Sift together flour, salt, and nonfat dry milk powder; mix with cornflake crumbs
Cut in butter until the mixture resembles coarse cornmeal. Combine egg and water; add to crumb mixture, stirring only until combined
Pat half of the dough into an ungreased 13x9 pan. Cover with sliced apples. Combine sugar and flour (and 1 tsp cinnamon, if desired); sprinkle over apples. Dot with butter.
On a lightly floured board, roll out the other half of the dough to fit the pan. Place over apples.
Bake in a 375-degree oven for about 30 minutes or until apples are tender.
While still warm, frosting with confectioners sugar icing. Cut into squares and serve warm or cold.