In a small saucepan, combine the cream, vanilla, and sea salt. Place over medium heat and bring to a simmer. Turn off the heat and cover to keep warm while you prepare the caramel.
In a large, stainless steel saucepan, whisk together the syrup and sugar and place over medium-high heat. Whisk until the sugar is fully melted. Clip a candy thermometer to the side of the pan, making sure the tip of the thermometer is immersed into the caramel mixture. Bring the mixture to a boil and cook until the temperature reaches 310°F (155°C).Avoiding stirring the caramel while it boils to avoid creating crystals, instead, very gently swirl the pan once or twice to even out the cooking of the caramel.
Remove the caramel from the heat. Slowly whisk the cream into the caramel. Be careful as the mixture will release hot steam and bubble up significantly. Make sure the candy thermometer still dips into the caramel; return to medium heat. Bring to a boil and cook until the temperature reaches 245 to 250°F (118 to 121°C) for a soft, chewy caramel, or 255 to 260°F (124 to 127°C) for a hard caramel.
Remove from the heat and whisk in the butter a few cubes at a time, incorporating fully before adding more. Once the butter is fully incorporated, pour the caramel into the prepared pan. Let cool for 10 minutes. Sprinkle with additional fleur de sel and let the caramel cool completely.
Remove the caramel from the pan by pulling on the parchment paper. Use a very sharp knife to cut into neat squares (lightly greasing or warming up the blade will help make this easier). Serve, or refrigerate the caramels in an airtight container for up to 1 week. (Make sure to separate layers with parchment or wax paper to avoid the caramels sticking to each other.) You can also wrap the caramels individually into squares of wax paper or in clear candy wrappers and package them in small bags or boxes to give them as gifts.
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