1) To make the crust: Whisk together the Perfect Pastry Blend or flour, salt, baking powder, confectioners' sugar, and buttermilk powder.
2) Add the shortening, working it in until the mixture is evenly crumbly.
3) Cut the butter into small (about ½") cubes. Add the butter to the flour mixture, and work it in roughly with your fingers, a pastry cutter, or a mixer. Don't be too thorough; the mixture should be very uneven, with big chunks of butter in among the smaller ones.
4) Add the 1 teaspoon vinegar and 4 tablespoons water, and toss to combine. Toss with enough additional water to make a chunky, fairly cohesive mixture. It should hold together when you gather it up and squeeze it in your hand.
5) Divide the dough into two pieces; one should represent about 40% of the dough, the other, about 60%. If you have a scale, this is easy; the smaller piece of dough should weigh about 8 3/4 ounces, the larger piece, about 13 1/4 ounces (if you've used about 7 to 8 tablespoons of water in the dough). If you don't have a scale and/or aren't good at math, eyeball it: the bottom crust needs to be larger than the top crust.
6) Shape each piece of crust into a rectangle; you're going to be rolling them into rectangles, so might as well give yourself a head start. Cover with plastic wrap, and refrigerate for 30 to 60 minutes, till thoroughly chilled.
7) Take the larger piece of pastry out of the fridge, and put it on a floured work surface. Roll it into an 11" x 15" rectangle. Don't worry about the ragged edges; they'll disappear under the top crust.
8) Place the crust in an ungreased 9" x 13" cake pan. Patch up any holes by pushing the pastry together with your fingers, or adding a pinch from the excess on the sides. Push the pastry up the sides of the pan a bit, to make a shallow pastry container for the apples.
9) Put the crust in the fridge while you get the apples ready. Start preheating your oven to 350°F.
10) Spread the bread crumbs or crushed cornflakes evenly over the crust.
11) Spread the sliced apples atop the crumbs.
12) Sprinkle the cinnamon-sugar over the apples.
13) Roll the remaining piece of pastry into a 9" x 13" rectangle. Again, don't worry too much about ragged edges.
14) Lay the top crust over the apples. Yes, apples will poke through. Seal the edges of the two crusts as well as you can. There'll be places where they don't quite meet. That's OK. If the whole thing has become warm and sticky and hard to work with, pop it in the fridge for 15 minutes to firm it up.
15) Just before baking, slash the crust 6 or 8 times to allow steam to escape. If desired, brush the crust with milk or cream, and sprinkle with coarse white sparkling sugar.
16) Bake the slab for an hour; it'll be golden brown, and the filling should be bubbling. Remove it from the oven, and allow it to cool a bit before serving. If you choose to add the glaze, let it cool completely.
17) To make the glaze: Combine the confectioners' sugar, boiled cider, cinnamon, salt, and enough milk or cream (if necessary) to make the mixture "drizzlable." Don't have boiled cider? Use plain milk or cream, maple syrup, honey, or thawed apple juice concentrate. Start with 1/4 cup of any of these; if you've made this kind of icing before, you know it's easier to add more liquid, than to try to take it away. Add enough liquid to make the glaze pourable.
18) Drizzle the glaze atop the slab.
Yield: 12 to 16 servings, depending on size.