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