In a large bowl, combine the flour, white sugar, and salt. Grate the cold unsalted butter using a cheese grater in batches to keep it cold, tossing the grated butter into the flour mixture as you go. Alternatively, cube or slice the butter and cut it into the flour using a pastry blender or forks. Make a well in the center and add 1/2 cup ice-cold water. Mix quickly with a fork, adding extra cold water 2 tablespoons at a time until the dough holds together without being sticky and still has visible small pieces of butter for flakiness.

Turn the dough onto a lightly floured surface and knead gently and quickly to bring it together. Divide the dough into two halves and flatten each into a 1-inch thick rectangle. Wrap each in plastic wrap and chill in the refrigerator for at least 30 minutes to relax gluten and maintain flaky layers.

Preheat your oven to 400°F (204°C). Lightly flour your work surface and roll out one dough rectangle to fit a 9×13-inch jelly roll pan. Press the rolled dough evenly into the bottom of the pan.

Spread the prepared apple pie filling evenly over the bottom crust. Roll out the second dough rectangle and place it on top of the filling. Use a rolling pin to help transfer the dough if needed. Pinch the edges lightly to seal and cut vents in the top crust to allow steam escape during baking.

Bake the assembled pie in the preheated oven for about 40 minutes, rotating the pan halfway through, until the crust is golden brown. Remove from oven and transfer to a wire rack to cool completely before glazing.

Beat together the confectioners’ sugar, room temperature butter, vanilla extract, and whole milk until smooth and creamy. Once the pie is cooled, spread the glaze evenly over the top. Allow the glaze to set and harden before slicing into squares and serving. Optionally, add a pinch of cinnamon or a squeeze of lemon juice to the glaze for extra flavor.