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 soft dough forms.
03 - Press dough evenly into prepared pan. Prick with fork and bake 18-20 minutes until lightly golden. Cool completely.
04 - Melt butter in heavy saucepan over medium heat. Stir in brown sugar, corn syrup, condensed milk, and salt. Bring to gentle boil, stirring constantly, and cook 7-10 minutes until thickened and golden (225°F on thermometer).
05 - Pour hot caramel evenly over cooled shortbread. Spread with spatula. Cool at room temperature 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. Spread evenly. Set at room temperature or refrigerate 30 minutes until firm. Lift from pan using parchment and cut into 16 bars with sharp knife, wiping blade between cuts.