Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, ensuring there is extra paper hanging over the edges.
Make the Shortbread Crust: In a large mixing bowl, beat together the cold butter and granulated sugar for 2-3 minutes until light and fluffy. Add the vanilla extract (if using) and mix until incorporated. Gradually add the flour and salt, mixing until the dough is crumbly. Press the dough evenly into the bottom of the prepared pan. Bake for 12-14 minutes or until golden. Remove from the oven and set aside.
Prepare the Apple Filling: Peel, core, and thinly slice the apples. In a large bowl, toss the apples with the flour and sugar until evenly coated. Arrange the apples over the baked shortbread crust.
Make the Crisp Topping: In a medium bowl, combine brown sugar, granulated sugar, flour, cinnamon, and salt. Add the cold butter cubes and use a pastry cutter or two knives to cut the butter into the dry ingredients until the mixture forms pea-sized crumbles. Sprinkle the topping evenly over the apples.
Bake: Place the pan in the oven and bake for 45-50 minutes, or until the topping is golden and the apple filling is bubbly. Remove from the oven and let cool completely before cutting into bars.
