Preheat the oven to 375 degrees F. Spray a 9×13-inch baking pan with cooking spray and set aside.
For the apple filling- place the diced apples in a large bowl and sprinkle with lemon juice. Mix gently. In a separate bowl whisk together flour, cornstarch, sugar, salt, cinnamon, nutmeg, and ground all-spice. Pour over the apples and carefully combine. Set aside.
For the dough-in a large bowl whisk together the flour, sugar, baking powder, salt, and cinnamon. Cut in the butter with a pastry blender or by using your hands, until butter is mixed in, but you still have small chunks of butter. Mix the egg and vanilla together. Add into the flour and butter mixture and combine. The dough will be crumbly. Pat half of the dough into the prepared pan.
Spread the apple mixture evenly over the bottom crust. Drizzle about 3/4 cup of salted caramel sauce over the apple filling. Crumble the remaining dough over the apple caramel layer and pat gently.
5. Bake for 45 minutes, or until the top is golden brown. Cool completely on a wire rack. Cut into squares. Drizzle salted caramel sauce over the squares and serve.
Note-you won't use all of the salted caramel sauce for this recipe. Store the leftover sauce in the refrigerator for up to 2 weeks.