01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Cream 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 double boiler or microwave in 20-second bursts. Stir until smooth.
07 - Pour melted chocolate over caramel layer. Spread evenly. Set at room temperature or refrigerate 30 minutes until firm.
08 - Lift bars from pan using parchment. Cut into 16 bars with sharp knife, wiping blade between cuts for clean edges.