01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Cream together butter and sugar until light and fluffy. Add flour and salt, mixing until a crumbly dough forms.
03 - Press dough evenly into the prepared pan. Bake for 18 to 20 minutes until lightly golden. Cool completely.
04 - Heat sugar in a medium saucepan over medium heat, stirring constantly until melted and amber. Add butter and stir until melted. Gradually stir in heavy cream and salt; cook for 1 to 2 minutes until smooth. Remove from heat and cool slightly.
05 - Pour caramel over the cooled shortbread base evenly. Refrigerate for at least 45 minutes until set.
06 - Melt chocolate and butter together in a heatproof bowl over simmering water or in the microwave, stirring until smooth.
07 - Spread melted chocolate evenly over the chilled caramel layer. Refrigerate for at least 30 minutes until firm.
08 - Lift bars from pan using parchment overhang. Cut into 16 bars with a sharp knife, wiping the blade between cuts for clean edges.