In a large mixing bowl, whisk together all-purpose flour, granulated sugar, and salt until combined. Cut in cold unsalted butter until the mixture resembles coarse crumbs.

Divide the dough into two equal portions, shape into discs, wrap with plastic wrap and refrigerate for about 30 minutes.

Prepare the filling by slicing Granny Smith apples, then combine with sugars, cornstarch, ground cinnamon, nutmeg, lemon juice, and vanilla extract in a large bowl.

Preheat oven to 375°F (190°C) and grease a 9x13-inch baking pan.

Roll out one disc of dough to about 1/8-inch thick, press into the prepared pan, and spread the apple filling over the crust.

Roll out the second disc of dough and place it over the apple filling, crimping the edges and cutting small vent holes.

Bake in the preheated oven for 45 to 50 minutes, until golden brown.

Remove from oven and allow to cool on a wire rack.

Whisk together sifted powdered sugar, milk, and melted butter until smooth for the glaze.

Drizzle the glaze over the cooled apple slices and let it set for about 15 minutes.

Cut into squares or rectangles, and serve your delicious treats.