Preheat and Prep: Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, leaving enough overhang on the sides to easily lift the bars out later. Lightly grease the parchment to prevent any caramel from sticking.
Make the Crust: In a large mixing bowl, combine the all-purpose flour, brown sugar, and melted butter. Stir with a fork or spatula until the mixture is well combined and resembles wet sand.
Pre-bake the Crust: Pour the crumbly dough into your prepared baking dish. Press it firmly and evenly into the bottom to form a solid crust. Bake for 10 to 12 minutes, or until the edges are lightly golden.
Add the Caramel: Remove the pan from the oven. While the crust is still hot, immediately pour the thick caramel sauce evenly over the top.
Add First Toppings: Sprinkle the chocolate chunks and chopped nuts (if using) evenly over the warm caramel layer.
Bake the Toppings: Return the pan to the oven and bake for an additional 10 minutes. Watch closely—you want the caramel to be gently bubbling and the chocolate chunks to be soft and melted.
Final Chocolate Layer: Remove the pan from the oven and immediately sprinkle the chocolate chips over the top. Allow the heat of the pan to soften them for a minute or two. You can leave them as chips for a textured look, or gently spread them into a smooth layer with a spatula.
Cool Completely: Allow the bars to cool completely in the pan at room temperature. For the cleanest slices, place the pan in the refrigerator for at least 1 hour after reaching room temperature before cutting them into squares.