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