For the shortbread, preheat the oven to 350°F (177°C). Grease an 8 x 8–inch (20 x 20–cm) square baking tin with baking spray, and line it with baking paper, leaving an overhang of baking paper over the edges so it can be easily lifted out. In a mixing bowl, combine the vegan butter and granulated sugar. Mix both ingredients using a stand mixer with a paddle attachment or an electric whisk on low speed for 2 to 3 minutes. Scrape down the sides as it is mixing to make sure it’s creamy and fully incorporated.

Add the vanilla extract, all-purpose flour, cornstarch and salt to the butter mixture. Mix on low speed for 1 to 2 minutes until the dough comes together. Press the dough into the prepared square baking tin. Use the back of a cup to level the dough, making sure it is even on all sides. Use a fork or a docker to dock the dough on all sides. Place the prepared baking tin in the oven, and bake a shortbread base for 10 to 12 minutes. Remove the baking tin from the oven, and let the shortbread base cool for 5 to 10 minutes in the baking tin. While the shortbread is baking, you can prepare the apple filling.

For the apple filling, wash, cut and core the apples on a cutting board. You can leave the skin on. Then cut the apples into small cubes about ⅓-inch (8-mm) dice. Transfer all the chopped apples into a clean bowl. Add the granulated sugar, all-purpose flour, cornstarch and cinnamon powder. Give it a good mix with a spatula. Transfer the apple mixture onto the prepared shortbread base. Press the apples with the spatula to make sure they are firmly packed on top of each other. Use your hands to spread an even layer of the crumble topping, and bake for 30 to 35 minutes or until the top is golden brown.

Remove the baking tin from the oven, and cool the bars in the tin before lifting out from the baking tin using the overhang. Drizzle caramel sauce on top of the bars if desired. Portion them into 9 square bars, and serve or store in an airtight container in the refrigerator for 2 to 3 days.