Preheat the oven to 300º.
Line an 8 x 8 baking pan with non-stick foil
Stir the melted butter, sugar, vanilla, and salt together in a medium bowl.
Add the flour and stir until everything is combined.
Press the mixture evenly into the prepared baking pan. Bake for 15 minutes.
While crust is baking, combine the sliced apples, flour, sugar & cinnamon and gently toss to combine. Set aside.
In another bowl mix the butter, oats, brown sugar, cinnamon, and flour together until crumbs.
Remove the crust from the oven, and increase the oven temperature to 350º.
Evenly layer the apples on top of the warm crust.
Crumble the streusel topping over the apples.
Bake for 30–35 minutes or until the streusel is lightly browned and apples are tender.
Let cool at room temperature for about a half hour, then chill for 2 hours.
Cut into bars and drizzle with caramel before serving.