01 - Line an 8 x 8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Melt 6.3 oz chocolate with 1 tbsp butter in a heatproof bowl over simmering water or in the microwave in 30-second intervals, stirring until smooth.
03 - Pour the melted chocolate into the prepared pan and spread evenly. Refrigerate 20 minutes until firm.
04 - In a medium saucepan over medium heat, melt granulated sugar, stirring constantly until golden-brown liquid forms (5–7 minutes).
05 - Add cubed butter and stir until fully melted and combined.
06 - Slowly pour in heavy cream while stirring (mixture will bubble). Stir in sea salt and vanilla. Simmer for 2–3 minutes until smooth and slightly thickened.
07 - Remove caramel from heat and cool 10 minutes. Pour evenly over chilled chocolate layer. Refrigerate 30 minutes until set.
08 - Melt remaining 4.2 oz chocolate with 1 tbsp butter in a heatproof bowl or microwave, stirring until smooth.
09 - Pour and spread evenly over caramel layer. Sprinkle flaky sea salt if desired. Chill at least 30 minutes until firm.
10 - Use parchment overhang to lift from pan, cut into 16 bars, and serve.