Preheat oven to 300 degrees F. Line an 8x8" square pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.

First, make your shortbread crust. Combine the melted butter, sugar, vanilla, salt, and flour in a medium bowl until a soft dough comes together. Press the shortbread crust into the bottom of the prepared pan. Bake for 15 minutes. Remove from the oven, but keep the oven on and increase the oven temperature to 350 degrees F.

While the crust pre-bakes, combine your apple filling ingredients into a medium bowl and toss to coat. When crust finishes pre-baking, top the crust with the apple mixture. It may seem like a lot of apples, but they will cook down some. Try to move the apple slices around to fill in any gaps and make an even layer of apples.

Next, make the crumble. Combine the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the cold, cubed butter with two forks or a pastry blender until the mixture resembles coarse, pea-size crumbs. Crumble the mixture evenly over the top of the apple layer.

Bake the pie bars for 30-35 minutes or until the top is golden brown and the apples are tender and set. Cool completely, then refrigerate for at least 2 hours, up to overnight, to set completely before cutting into squares to serve. Drizzle with salted caramel before serving.