Preheat your oven to 350°F. Lightly grease a 7x5-inch baking dish and line with parchment paper (parchment paper is optional, but it's nice to be able to lift the bars out of the dish to cut them).
In a medium bowl, combine butter, powdered sugar, and granulated sugar, and beat until light and fluffy, 1 to 2 minutes. Beat in vanilla. Add flour and salt, and beat until well mixed.
Turn mixture out into your prepared baking dish and press into an even layer. Bake for 18 to 21 minutes, until lightly browned.
Apple Layer
While your shortbread bakes, combine diced apples, sugar, flour, cinnamon, and lemon juice. Stir together and set aside.
Brown Sugar Streusel
In a small bowl, combine flour, brown sugar, and butter, and use a fork or your fingers to mix until the mixture begins to clump and crumbs form. Place streusel in the refrigerator until the timer goes off.
Once the shortbread is lightly golden, remove from the oven and spread the apples over the top. Remove streusel from the fridge and drop in crumbles over the apples. Bake for an additional 15 to 20 minutes until streusel is lightly browned and apple filling is bubbling slightly.
Cool completely before cutting and enjoy!
Notes
Because of the moisture of the apples, the shortbread in these bars will lose its crispness overnight. If you won't finish the bars on the first day, I recommend freezing them. They freeze very well, and when you are ready to eat one, just remove from the freezer and allow to sit at room temperature for 15 to 20 minutes.Approximate nutritional information is for 8 bars.