01 - Preheat oven to 350°F and line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Cream together softened 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. Allow to cool completely.
04 - Heat sugar over medium heat in a saucepan, stirring constantly until melted and amber. Add butter and stir until melted, then slowly add heavy cream and salt, stirring until smooth. Simmer for 1 to 2 minutes, then remove from heat and allow to cool slightly.
05 - Pour caramel evenly over the cooled shortbread base and refrigerate for at least 45 minutes until set.
06 - Melt chocolate and butter together in a heatproof bowl over simmering water or using short microwave bursts, stirring until smooth.
07 - Spread melted chocolate evenly over chilled caramel layer and refrigerate for at least 30 minutes until firm.
08 - Remove bars from pan using parchment overhang. Cut into 16 bars with a sharp knife, wiping blade between cuts for clean edges.