Prepare the Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, baking powder, and kosher salt.

Cut in the Fat: Add the cold, cubed butter and the cold shortening to the bowl. Using a pastry blender or two forks, cut the fats into the flour until the mixture resembles coarse peas.

Mix the Wet Ingredients: In a small measuring cup, lightly whisk together the egg yolks, fresh lemon juice, and ice water.

Form the Dough: Pour the wet mixture over the flour and gently toss with a fork until the dough just begins to pull together. Do not overmix. Divide the dough into two equal halves, flatten them into thick rectangles, wrap tightly in plastic wrap, and chill in the refrigerator for at least 30 minutes.

Preheat and Prep: Preheat your oven to 400°F (200°C). Have an ungreased 9x13-inch jelly roll pan ready.

Roll the Bottom Crust: On a well-floured surface, roll one half of the chilled dough into a rectangle slightly larger than your pan. Gently transfer the dough to the pan, pressing it lightly into the bottom and slightly up the sides. Patch any tears with your fingers.

Add the Filling: Spread the apple pie filling in an even layer over the bottom crust, pushing it all the way to the corners using an offset spatula.

Add the Top Crust: Roll out the second half of the dough to the same rectangular size. Carefully drape it over the apple filling. Pinch the top and bottom crust edges together to seal. Cut 5 to 6 small slits in the top crust with a sharp knife to allow steam to escape.

Bake: Bake on the center rack for 35 to 40 minutes, rotating the pan halfway through, until the top crust is a beautiful, deep golden brown. Remove from the oven and place on a wire rack.

Make the Glaze: While the pastry is cooling, whisk together the powdered sugar, softened butter, vanilla extract, and milk until completely smooth. Add the milk one tablespoon at a time until you reach a smooth, spreadable consistency.

Glaze and Cool: While the pastry is still slightly warm (but not burning hot), pour the glaze over the top and spread it evenly edge-to-edge.

Slice and Serve: Let the pastry cool completely so the glaze hardens and sets. Slice into squares and enjoy!