Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, leaving some overhang for easy removal later.

To make the shortbread base, cream together the butter and sugar until light and fluffy. Add in the flour and salt, mixing until a soft dough forms.

Press the dough evenly into the bottom of your prepared baking dish. Prick lightly with a fork and bake for 20–25 minutes, or until lightly golden. Remove and let cool completely.

For the caramel layer, combine the butter, brown sugar, corn syrup, and sweetened condensed milk in a medium saucepan over medium heat. Stir constantly as it melts, then increase the heat to bring it to a gentle boil.

Continue stirring and let the mixture bubble gently for 5–7 minutes, until it thickens and turns a golden caramel color. Pour the caramel over the cooled shortbread and spread evenly. Let cool for 15 minutes, then refrigerate for at least 1 hour.

To make the chocolate topping, melt the chocolate chips with the butter (if using) in the microwave in 20-second bursts, stirring between each, until smooth. Pour over the chilled caramel layer and spread evenly.

Chill the entire dessert until the chocolate is set, at least another hour. Use a warm knife to slice cleanly into squares or bars.