Preheat the oven to 350°F (175°C) and prepare a 9x13 inch baking dish.
In a large bowl, cream together the softened butter and brown sugar until light and fluffy.
Add the flour, baking powder, and salt to the creamed mixture. Mix until the dough forms.
Press the remaining dough evenly into the bottom of the prepared baking dish.
Bake the base in the preheated oven for 15 minutes, until lightly golden.
While the base is baking, prepare the apple filling. In a separate bowl, combine the diced apples, cinnamon, and nuts (if using).
Remove the crust from the oven and spread the apple mixture over the top.
Drizzle the caramel sauce over the apples, spreading it evenly with a spatula.
Sprinkle the reserved crumb topping evenly over the caramel layer.
Bake for an additional 25-30 minutes, or until the topping is golden brown.
Once baked, remove from the oven and let cool completely before slicing.