Preheat oven to 400 degrees. Grease a 12x 17 jelly roll pan.
In a large bowl combine all of the ingredients for the crust, except the cornflakes. Using a pastry cutter or a fork work everything together (like you would a pie crust). Once everything is combined divide the dough in half. Using your fingers press 1/2 of the dough into the prepared pan. It will be thin that is ok. Sprinkle the corn flakes evenly over the crust.
In a separate bowl toss the apples, sugar and cinnamon together. Once well coated, spread the apples evenly over the crust. Cover the apples with the remaining crust. It can be a little sticky, so I roll out small portions and place it on top. Kind of like patchwork.
Bake for 10 minutes at 400 degrees. Turn down to 350 degrees and bake for 40 more minutes. Remove from the oven and allow to cool. Once cool you can frost, if using.
For the frosting mix everything together, and adjust the amount of milk to get a thin and almost drizzle-able frosting.