In a large bowl, stir together the flour, sugar, and salt.
Remove one stick of butter from the fridge and using a cheese grater, grate butter over the flour mixture. If butter gets too soft, place in the freezer until firm again and start on the next stick of butter. Continue grating until all butter is used, stirring into the flour mixture in between sticks.
(Alternatively, you can cube or slice the butter and cut in using a pastry blender or two forks.)
Create a well in the center of the flour mixture and add the 1/2 cup ice water. Using a fork, toss quickly to combine.
Refill ice water container, then drizzle in 2 tablespoons of cold water at a time, stirring after each addition, until dough is moist and holds together easily. Try not to add any more water than you need to — dough shouldn’t be sticky. You should be able to see small pieces of butter throughout the dough.
On a lightly floured work surface, quickly and gently knead dough just to bring it together. Divide in half, then flatten each half into a 1-inch thick rectangle.
Wrap each rectangle in plastic wrap and chill at least 30 minutes.
When ready to bake, preheat oven to 400°F and set aside a 9×13-inch jelly roll pan.
Lightly dust a work surface with flour, then roll out one rectangle of dough roughly the same size as jelly roll pan. Press into the bottom of the pan.
Spread apple filling over the pie dough.
Roll the remaining rectangle of dough out to the same size, then pick up and lay on top of the apple filling. (You can also use a rolling pin to roll the dough up to make transferring easier.) Pinch edges of dough together but don’t worry too much about sealing completely.
Slice a few vents in top crust to release steam, then bake until crust is golden brown, about 40 minutes, rotating pan halfway through. Remove to a wire rack to cool.
When pie has cooled, make the glaze:
Beat together the sugar, butter, vanilla, and milk until smooth, then spread glaze over the top of the crust evenly. Let set and harden, then slice into squares and serve. Enjoy!