Preheat the oven to 350 F (177 C). Line an 9 x 9 pan with parchment paper along the bottom and two sides.
Peel, core and dice the apples into ½ inch sized cubes. Add the diced apples, sugar, light brown sugar, apple pie spice, cornstarch, vanilla extract, lemon juice and salt to a large mixing bowl. Toss the ingredients together until the apples are fully coated. Set aside while making the shortbread crust and crumble topping. 4 medium apples, ¼ cup (50 g) granulated sugar, ¼ cup (50 g) light brown sugar, 2 teaspoons apple pie spice, 1 tablespoon cornstarch, 2 teaspoons (10 ml) vanilla extract , 1 teaspoon lemon juice, ¼ teaspoon salt
Make the buttery crust and crumb topping by combining the flour, sugar, light brown sugar, baking powder, cinnamon or apple pie spice and salt together in a food processor. Pulse 2 -3 times to combine the ingredients.
2¼ cups (270 g) all purpose flour, ½ cup (100 g) granulated sugar, ¼ cup (50 g) light brown sugar, 1 teaspoon cinnamon or apple pie spice, ½ teaspoon baking powder, ½ teaspoon salt
Pulse in cubes of cold butter until the mixture resembles a crumble texture. Finally pulse in the egg yolk and vanilla extract until the mixture starts to clump together.
¾ cup (170 g) cold cubed butter, 1 large egg yolk , 1 teaspoon vanilla extract
Evenly press ⅔ of the shortbread mixture into the prepared pan. Using a slotted spoon, transfer the apple filling onto the shortbread crust. Top the apple pie filling with the remaining crumble mixture. Make sure to cover as much of the filling as much as possible, but spots of exposed filling is fine.
Bake the apple pie bars for 45 - 55 minutes or until the crust is a nice golden brown with edges of the crust pulling slightly away from the sides of the pan. The apple filling will thicken and bubble.
Cool the apple crumble bars in the pan for 1 hour. If the bars are too soft to easily transfer from the pan after 1 hour at room temperature, transfer to the fridge to set for another hour. Once the bars are set, top with powdered sugar, maple glaze salted caramel sauce or enjoy as is.