Prepare the oven and pan. Preheat the oven to 350°F (175°C). Grease the bottom and sides of a 9×13-inch (23×33 cm) baking pan.
Mix the crust. In a large bowl, beat the softened butter, granulated sugar, and brown sugar with an electric mixer until creamy. Add the flour and salt. Mix until a crumbly dough forms that holds together when pressed.
Prebake the crust. Press the dough evenly over the bottom and up the sides of the prepared pan. Bake for 20 minutes, until lightly golden.
Prepare the filling. While the crust bakes, combine the chopped apples, brown sugar, cinnamon, vanilla extract, and salt in a bowl. Toss until evenly coated, then spread over the partially baked crust.
Make the crumble. In a medium bowl, stir together the flour, chopped walnuts, brown sugar, and salt. Add the melted butter and mix with a fork until crumbly clumps form.
Add the topping and bake. Sprinkle the crumble evenly over the apple filling. Bake for about 1 hour, until the topping is golden brown and the apples are tender. Loosely cover with foil if the topping browns too quickly.
Cool and serve. Let the bars cool completely in the pan before cutting into 16 portions. Drizzle with caramel sauce just before serving, if desired.
