Preheat oven to 350°F (175°C) and line a 9×13-inch baking dish with parchment paper.
Combine flour, brown sugar, and melted butter until crumbly.
Press firmly into the prepared pan.
Bake for 10–12 minutes until lightly golden.
Spread caramel evenly over the warm crust.
Sprinkle chocolate chunks and chopped nuts over the caramel.
Bake for another 10 minutes until bubbly.
Remove from the oven and immediately sprinkle chocolate chips over the top.
Let the chips soften, then spread if desired.
Sprinkle with flaky sea salt if using.
Cool completely.
Refrigerate for at least 1 hour before slicing.