Preheat your oven to 300 degrees. Grease an 8×8 baking dish with cooking spray. Set aside.
In a stand mixer fitted with a paddle attachment, beat the butter on high until it has a soft white hue, and it becomes creamy. Next, add in the sugar, vanilla, and salt. Mix until well combined. Then, add the flour and blend until you have a soft dough.
Place the dough in the bottom of the prepared baking dish. Using your hands, gently press the mixture into the bottom of the pan to form an even crust — bake in the preheated oven for 15 minutes.
Wash and dry your apples. Peel and core your apples, then thinly slice. In a medium bowl, toss the sliced apples with flour, sugar, cinnamon, and nutmeg until well coated. Set aside.
Remove the crust from the oven. Preheat the oven to 350 degrees. Then in a small bowl mix the flour, brown sugar, and melted butter. A crumb-like mixture will form.
Sprinkle half of the crumb mixture on to the bottom of the shortbread. Next, add the apples. Spread the apples out evenly, and then sprinkle the remaining crumb mixture on top of the apples — bake for 30 minutes or until golden brown.
Allow the apple crisp shortbread bars to cool on the counter completely before placing them into the fridge to set for 2 hours or overnight. Once set, cut into 9 bars. If you prefer them warm, be aware that they may be fragile when you cut into them straight out of the oven.
Check out what types of apples are best for baking.