01 - Preheat oven to 350°F (175°C). Line a 9x13-inch (23x33 cm) baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, cream together the softened butter and sugar until light and fluffy. Mix in flour and salt until a soft dough forms.
03 - Press the dough evenly into the prepared pan. Prick with a fork. Bake for 18–20 minutes, or until lightly golden. Remove from oven and let cool completely.
04 - In a heavy saucepan over medium heat, melt the butter, then stir in brown sugar, corn syrup, sweetened condensed milk, and salt. Bring to a gentle boil, stirring constantly, and cook for 7–10 minutes until thickened and golden (mixture should reach 225°F/107°C on a candy thermometer).
05 - Pour the hot caramel evenly over the cooled shortbread. Spread with a spatula. Allow to cool and set at room temperature for 1 hour, or until firm.
06 - Melt chocolate and butter together in a heatproof bowl set over simmering water (double boiler) or in the microwave in 20-second bursts. Stir until smooth.
07 - Pour the melted chocolate over the caramel layer. Spread evenly. Let the bars set at room temperature or refrigerate for 30 minutes until the chocolate is firm but not hard.
08 - Use the parchment overhang to lift the bars from the pan. Cut into 16 bars with a sharp knife, wiping the blade between cuts for clean edges.