Line a 8x8 inch (or 9x9 inch pan) square baking pan with parchment paper and grease lightly for easy removal later.
Process 10.5 ounces shortbread cookies to crumbs, add to a medium-large mixing bowl. Add 7 tablespoons unsalted butter (melted) and stir until fully combined (should be texture of wet sand). Press shortbread mixture into prepared pan in an even layer. Refrigerate until firm.
Add 6 tablespoons light brown sugar, 6 tablespoons unsalted butter, 1 (14-oz) can sweetened condensed milk and ¼ teaspoon salt to a small saucepan and bring to a boil. Reduce heat to low and simmer until thickened while stirring often.
Pour caramel over hardened shortbread base and refrigerate again until firm.
Melt 8 ounces semi-sweet chocolate chips together with 1 tablespoon unsalted butter until melted and glossy but still spreadable.
Pour chocolate over the caramel layer. It looks lovely when the chocolate is spread with a spatula in a way that it isn't completely smooth, but showing some chocolate swirls and some texture (make sure you do not disturb the caramel layer).
Allow to cool at room temperature until hardened. Lift from pan and cut into small bars.
