Yield: 12 bars.
For the crust:
1 cup all-purpose flour
2 tablespoons cornstarch
¼ cup granulated sugar
1 teaspoon vanilla extract
¼ teaspoon salt
½ cup unsalted butter, melted
For the filling:
3 large McIntosh apples, peeled, cored and thinly sliced
2 tablespoons lemon juice
1 tablespoon all-purpose flour
2 tablespoons granulated sugar
1 teaspoon ground cinnamon
¼ teaspoon ground nutmeg
For the topping:
¼ cup old-fashioned oats
⅓ cup packed dark brown sugar
¼ teaspoon ground cinnamon
½ cup all-purpose flour
¼ cup unsalted butter, cold and cubed
For the caramel sauce:
1 cup sugar
6 tablespoons salted butter, cut into 6 pieces
½ cup heavy cream
1 teaspoon salt
For the crust: Preheat the oven to 300°. Line the bottom and sides of an 8-inch square baking pan with parchment paper, leaving overhang on all sides. Set aside.
Add the flour, cornstarch, granulated sugar, vanilla, and salt to the mixing bowl of a food processor. Pulse 3 to 5 times. Add melted butter and pulse again until mixture resembles coarse sand. Press the mixture evenly into the prepared baking pan. Bake for 15 minutes while you prepare the filling and topping.
For the filling: Combine the sliced apples, lemon juice, flour, granulated sugar, cinnamon, and nutmeg together in a large bowl and toss until apples are evenly coated. Set aside.
For the topping: Whisk the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the chilled butter with a pastry blender until the mixture resembles coarse crumbs. Set aside.
Remove the crust from the oven, and increase temperature to 350°. Pour apple filling over crust and press tightly, ensuring apples are packed into the crust. Sprinkle the apple layer with the topping and bake for 30 to 35 minutes or until the topping 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 (or overnight). Lift the parchment out of the pan using the overhang on the sides and cut into 12 bars. Drizzle salted caramel sauce on top of each bar and store in an airtight container in the refrigerator. Serve warm, at room temperature, or straight out of the refrigerator.
For the caramel sauce: Heat sugar in a medium saucepan over medium heat, stirring constantly until sugar melts into a thick, amber-colored liquid. Be careful not to burn. Once sugar is completely melted, immediately, and carefully, add the butter. The caramel will bubble rapidly when the butter is added.
Stir the butter into the caramel until it is completely melted, about 2 to 3 minutes.
Slowly drizzle in the heavy cream while stirring. Note: The mixture will rapidly bubble when cream is added. Allow the mixture to boil for 1 minute. Remove from heat and stir in the salt. Allow to cool before using. Caramel sauce can be made ahead.