• Preheat the oven to 300F degrees. Line an 8-inch baking pan with aluminum foil, leaving an overhang on all sides. Set aside.

Make the crust:

• Stir the melted butter, granulated sugar, vanilla, and salt together in a medium bowl. Add the flour and stir until combined. Press the mixture evenly into the prepared baking pan. Bake for 15 minutes.

Make the apple filling:

• Combine the sliced apples, flour, granulated sugar, cinnamon, and nutmeg together in a large bowl until all of the apples are evenly coated. Set aside.

Make the streusel:

• Mix together the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the chilled butter with a pastry blender or two knives (or even with your hands) until the mixture resembles coarse crumbs. Set aside.
• Remove the crust from the oven, and turn the oven up to 350F degrees. Evenly layer the apples on top of the warm crust. Layer them tightly and press down to fit. Sprinkle the apple layer with streusel and bake for 30–35 minutes or until the streusel is golden brown.
• Remove from the oven and allow to cool for at least 20 minutes at room temperature, then chill in the refrigerator for at least 2 hours. Lift the foil out of the pan using the overhang on the sides and cut into bars. Once cut, pour salted caramel sauce on top. Enjoy these apple pie bars warm or cold. The bars will stay fresh in an airtight container in the refrigerator for 3 days or frozen up to 3 months - thaw overnight in the fridge, then drizzle with salted caramel before serving.