Preheat your oven to 350 degrees F. Line a 9x13-inch baking pan with parchment paper, leaving a couple inches of overhang on the long sides so you can lift the bars out later. Grease the parchment lightly with cooking spray or butter.

Make the shortbread base: In a large bowl, whisk together 2 cups flour, powdered sugar, and 1/4 teaspoon of the salt. Add the cold butter pieces and vanilla extract. Use a pastry cutter or your fingertips to work the butter in until the mixture forms coarse clumps that hold together when pressed. It should look like wet sand. Press this mixture firmly and evenly into the bottom of the prepared pan. Use the flat bottom of a measuring cup to compact it. Bake for 12 minutes until the edges are just starting to look set and dry. Pull it out and set aside.

While the base bakes, make the apple filling: Combine the diced apples, granulated sugar, cinnamon, allspice, cornstarch, lemon juice, and the remaining 1/4 teaspoon salt in a medium saucepan over medium heat. Stir to coat the apples. Cook for 7 to 9 minutes, stirring often, until the apples are softened and most of the liquid has cooked off. The mixture should look jammy but not watery. If it looks too wet, cook another 2 minutes. Remove from heat and let it cool for 5 minutes.

Make the crumble topping: In a large bowl, combine the oats, 3/4 cup flour, and brown sugar. Add the cold butter pieces and work them in with your fingertips until the mixture forms irregular crumbles ranging from pea-sized to almond-sized. You want some bigger chunks in there. Stir in the butterscotch chips.

Spread the cooked apple filling in an even layer over the par-baked shortbread base. Scatter the crumble topping evenly over the apples, covering them completely. Gently press the crumble down just a little so it does not slide off when baked.

Bake for 30 to 35 minutes until the crumble topping is golden brown and you can see the apple filling bubbling around the edges. The butterscotch chips will have melted and re-set into the crumble. Let the pan cool on a wire rack at room temperature for at least 30 minutes, then transfer to the refrigerator and chill for at least 1 hour before cutting.

Use the parchment overhang to lift the whole slab out of the pan onto a cutting board. Cut into 16 bars using a sharp knife. Wipe the knife clean between cuts for neat edges. Serve at room temperature.