Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: For the shortbread crust, in a large bowl, cream together 1 cup (2 sticks) softened unsalted butter and granulated sugar until light and fluffy.
Step 3: Gradually add the all-purpose flour and 1/4 teaspoon salt, mixing until just combined and a soft dough forms.
Step 4: Press the shortbread dough evenly into the bottom of the prepared baking pan. Bake for 20-25 minutes, or until the edges are lightly golden. Remove from oven and let cool slightly.
Step 5: While the crust cools, prepare the caramel. In a heavy-bottomed saucepan over medium heat, combine the remaining 1/2 cup (1 stick) unsalted butter, light brown sugar, sweetened condensed milk, and golden syrup (if using).
Step 6: Stir constantly until the mixture comes to a boil, then reduce heat to low and continue to simmer, stirring frequently, for 5-7 minutes, or until the caramel thickens and turns a rich golden brown. Be careful not to burn it.
Step 7: Remove the caramel from the heat and stir in the vanilla extract and 1/2 teaspoon sea salt. Pour the hot caramel evenly over the baked shortbread crust.
Step 8: Place the pan in the refrigerator for at least 30 minutes to allow the caramel to set.
Step 9: For the chocolate topping, melt the semi-sweet chocolate chips (and optional 2 tablespoons butter) in a microwave-safe bowl in 30-second intervals, stirring until smooth, or use a double boiler.
Step 10: Pour the melted chocolate over the chilled caramel layer, spreading it evenly with an offset spatula. You can optionally sprinkle a little extra sea salt on top for a finishing touch.
Step 11: Return the pan to the refrigerator for at least 2 hours, or until the chocolate is completely set and firm.
Step 12: Once fully chilled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place on a cutting board. Run a sharp knife under hot water, wipe it dry, and then carefully cut the bars into squares. Reheat and wipe the knife after each cut for the cleanest edges.