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1
Line an 8x8-inch square baking tin with parchment paper (allowing some of the parchment paper to hang over the sides) and generously coat with nonstick cooking spray. Set aside.
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2
Combine cream, corn syrup, vanilla extract, and kosher salt in a small saucepan over medium heat and bring to a simmer. Remove from heat and set aside.
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3
Add sugar and 3/4 cup water to a large, heavy-bottomed saucepan fit with a candy thermometer. Do not stir. Cook over medium heat until the mixture develops a dark amber color and the thermometer reads 370u00b0F. If sugar starts to build up on the sides of the pot, use a wet pastry brush to dissolve the crystals. Do not stir during cooking. This process will take about 15 minutes, and you may need to brush the sides of the pot up to 10 times to remove sugar buildup.
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4
Remove pan from the heat and slowly and carefully add the balsamic vinegar while stirring with a spoon or heat-resistant spatula. The mixture will violently bubble and temporarily seize up. Stir for an additional 30 seconds until the sugar remelts.
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5
Add the cream mixture to the pot and stir for another 30 seconds until smooth. Cook the mixture over medium heat without stirring until a candy thermometer reaches 260u00b0F. This can take up to 20 minutes. The sides of the pot will begin to darken, but there is no need to use a wet pastry brush to remove buildup at this point.
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6
Pour mixture into prepared baking tin and let set at room temperature overnight. Once firm, remove the caramels from the pan by lifting the parchment paper. Sprinkle with flaky sea salt and cut into a 9x6 grid resulting in 54 rectangular pieces using a chef's knife coated with nonstick cooking spray. (If the caramel is too sticky, pop it in the fridge for 15 minutes and try again.)
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7
Wrap each piece in a square of parchment paper slightly bigger than the caramel. Store at room temperature.
