Preheat your oven to 350°F. Line an 8x8 baking pan with foil or parchment paper and grease it lightly.

In a large mixing bowl, combine granulated sugar, brown sugar, 1 ½ cups of flour, baking powder, and salt; whisk to combine.

Add the chilled unsalted butter, mixing until the mixture resembles coarse crumbs.

Pour in the vanilla extract and oats, then add ground cinnamon; mix gently. Reserve 1 cup of this mixture for topping.

Press the remaining mixture into the bottom of the prepared pan to form a crust. Bake the crust for about 10 minutes.

In a large bowl, toss the thinly sliced apples with 1 tablespoon of flour, 2 tablespoons of granulated sugar, 1 teaspoon of cinnamon, and lemon juice until evenly coated.

Once the crust is set, layer the apple mixture evenly over the warm crust.

Sprinkle the reserved crumb mixture over the apples.

Bake for 30-35 minutes until the top is golden brown and the filling is bubbling.

Allow cooling in the pan for 10-15 minutes, then lift out using the foil or parchment paper and transfer to a cooling rack to cool completely before slicing.

Serve by cutting the bars into squares and drizzling with additional caramel sauce.