01 - Line an 8 x 8 inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Melt chocolate and butter together in a heatproof bowl over simmering water or in the microwave in 30-second intervals, stirring until smooth.
03 - Pour half of the melted chocolate into the prepared pan, spreading evenly, then chill in the refrigerator for 15 minutes until set.
04 - Combine caramels and heavy cream in a saucepan over low heat, stirring continuously until smooth and melted.
05 - Spread the caramel evenly over the chilled chocolate layer, sprinkle with chopped nuts, pressing gently, and optionally add flaky sea salt.
06 - Pour remaining chocolate over the nut layer, spreading to fully cover.
07 - Refrigerate the pan for at least 30 minutes until fully set.
08 - Lift the slab from the pan using the parchment overhang and cut into 12 bars with a sharp knife. Serve at room temperature or chilled.