preheat oven to 400F. Line a baking sheet with a piece of parchment paper and set aside.
To make the apple filling, add the apples, sugar, cinnamon, vanilla, salt, cornstarch, and cider (or water) to a large heavy bottomed saucepan. Heat over medium heat until bubbling, and allow to gently simmer for about 10 minutes. You want the apples to be tender, but not falling apart.
Set aside to cool until it is lukewarm. You can put it in the refrigerator to speed the process.
Roll out one of the puff pastry sheets lengthwise until it is about 14 inches long. Place on the prepared baking pan.
Roll out the other sheet of puff pastry to the same size. Using a sharp knife, make 8-10 slashes in the pastry, cutting all the way through.
Spread the cooled apple filling evenly over the bottom sheet of puff pastry, leaving about 3/4" on all sides for crimping. Dot the butter evenly over the filling.
Place the top layer of puff pastry over the apple filling. Use a fork to crimp the edges together.
Brush the whole top with the milk.
Bake for about 30-35 minutes until puffed and golden brown
Remove from the oven and allow to cool to room temperature before glazing.
To make the glaze, whisk the powdered sugar with the milk until smooth. Drizzle evenly over the strudel, you may have some glaze leftover.
Store leftover strudel loosely covered on the counter.