Preheat the oven to 300 degrees. Line an 8 x 8 baking dish with foil and set aside.
In a medium bowl combine the butter, sugar, vanilla, and flour. Mix until it forms a dough.
Press the dough into the prepared pan and bake for 15 minutes. Remove from oven and raise the oven temperature to 350 degrees.
In a medium bowl toss the apples with the flour, brown sugar, cinnamon, and salt.
Layer the apple slices on top of the baked shortbread crust.
In a small bowl combine the oats, brown sugar, flour, cinnamon, butter, and 1 teaspoon of salt. Mix until a crumble forms. Sprinkle over top of the apples.
Place the bars back into the oven and bake for 30 minutes.
Remove from the oven and refrigerate for at least 2 hours.
When ready to serve use the foil to help remove the bars from the pan and place on a cutting board. Cut into 12 bars.
Serve with a drizzle of caramel and a sprinkle of sea salt. Store remaining bars without the caramel sauce in an air tight container for 3-4 days.