"Invited to a retirement picnic honoring a good friend who loves cherry pie. Wanted to honor him by bringing cherry pie suitable for a picnic. Couldn't find a recipe for cherry pie bars, so put one together!"
Beat 1/2 cup milk and egg yolk together in a bowl. Combine flour and salt in a separate bowl. Cut butter into flour mixture using a pastry blender or fork until mixture resembles coarse crumbs; stir into milk-egg mixture until a ball of pastry dough forms. Separate dough into 2 pieces.
Roll 1/2 of the dough on a floured work surface large enough to fit in the bottom and up the sides of a 10x15-inch jelly roll pan. Place dough on pan. Sprinkle crushed corn flakes over dough.
Drain 1/3 cup liquid from each can of cherry pie filling and reserve liquid for another use. Carefully spoon cherries over corn flakes layer.
Roll remaining dough into an 11x16-inch rectangle on a floured work surface; place on top of cherry layer. Tuck 1/2 inch of dough under the bottom crust and gently seal edges.
Beat egg white in a bowl until foamy; brush over top dough layer. Sprinkle white sugar over dough.
Bake in the preheated oven until lightly browned, about 1 hour. Transfer to a cooling rack to cool completely, about 3 hours.
Whisk confectioners' sugar, 2 tablespoons milk, and vanilla extract together in a bowl until glaze is smooth. Spoon glaze into a plastic bag and snip 1 corner. Slice bars and squeeze glaze onto each bar.