Preheat the oven to 300°F. Line a 9x9-inch pan with parchment paper. Set aside.
For the crust, in a medium bowl, stir together the melted butter, light brown sugar, salt, and vanilla. Add the flour. Stir until well combined.
Spread the batter evenly into the prepared pan.
Bake for 15 minutes and then remove from the oven.
Make the apple filling. In a large bowl, combine the sliced apples, vanilla, cinnamon, light brown sugar, and flour. Stir until all of the apple slices are evenly coated.
For the crumble topping, in a medium bowl, combine the melted butter, cinnamon, and sugar. Add the flour and salt. Mix with a fork until crumbly. Set aside.
Layer the apples on top of the warm crust. Press them down to layer them tightly. Top with crumb mixture.
Bake for 30–35 minutes until golden brown.
Remove from oven and allow to cool for 20-30 minutes at room temperature. Then chill in the fridge for at least 1 hour or overnight. Cut into 9 bars and drizzle each bar with caramel topping.