Preheat oven to 350 degrees F. Line a 9x13-inch baking pan with foil and spray with non-stick cooking spray.
Make the Crust: In a medium bowl, stir together the graham cracker crumbs, brown sugar, cinnamon and salt. Add the melted butter and stir with a fork until the mixture is evenly moistened. Turn the mixture out into the prepared pan and press into an even layer on the bottom of the pan.
Assemble the Bars: On top of the crust, sprinkle the chocolate chips, caramels, chopped apple, and walnuts (in that order). Drizzle the sweetened condensed milk evenly over the entire mixture. Using the back of a spoon, gently press down on the mixture so that most of it is at least partially submerged in the milk.
Bake the Bars: Bake until the edges of the bars are golden brown and the center is set, about 30 to 35 minutes. Allow to cool completely in the pan, then remove and slice into squares. The bars can be stored at room temperature in an airtight container for up to 5 days.
Notes
You can buy the boxed graham cracker crumbs or crush your own. I find that running them through the food processor makes the best crumbs, versus crushing them with a rolling pin or something similar.
I used Granny Smith apples because I think the tartness contrasts really well with the other sweet components of the bars, but feel free to use your favorite apple, or whatever you have on hand.
While I used walnuts for the recipe, feel free to use whatever nuts you'd like - pecans would be great, too, and if we didn't have a peanut allergy in our house, you can bet that would have been my #1 choice!