01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - 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 and bake for 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 for 7-10 minutes until thickened and golden (225°F on candy thermometer).
05 - Pour hot caramel evenly over cooled shortbread. Spread with spatula and cool at room temperature for 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. Lift from pan using parchment and cut into 16 bars, wiping knife between cuts.