Use a candy thermometer to accurately reach 240 degrees (F) for proper setting, and allow the mixture to cool to 120 degrees (F) before pouring to achieve the intended texture. Line an 8x8 pan with parchment for easy removal and sprinkle the reserved flaky sea salt on top just before chilling. Store in an airtight container in the refrigerator; fudge will keep up to 7 days.