Line the bottom and sides of an 8-inch square baking dish with parchment paper. Lightly butter the parchment.
In a small saucepan or a heatproof measuring cup, combine the cream, butter, vanilla extract, vanilla bean seeds, and fleur de sel. Heat over medium-high heat and bring to a boil. (Alternatively, microwave until the butter is melted and the cream has come to a boil.) Remove from the heat and set aside.
In a medium saucepan, combine the sugar, corn syrup, and water. Heat over medium-high heat, stirring occasionally until the sugar is dissolved. Boil, without stirring but gently swirling the pan occasionally, until the mixture is a light golden caramel color.
Once the sugar has reached the desired amber color, carefully stir the warm cream mixture into the caramel in a steady drizzle down the inside of the pan – the mixture will bubble up, so pour slowly and stir constantly.
Continue simmering the mixture until it registers exactly 248˚ F on a candy thermometer*.
Immediately remove from the heat and pour into the prepared pan. Let cool for 30 minutes, then sprinkle lightly with additional fleur de sel. Continue to let sit until completely set and cooled. Cut into 1-inch pieces (a buttered pizza cutter works well). Wrap the individual caramels in small pieces of wax or parchment paper, about 4-inch squares.
*The type of candy thermometer that clips to the side of the pan is really important here. My Thermapen is pictured above because my new candy thermometer is en route after my old one bit the dust, but I strongly urge you to use the clip on kind. They are inexpensive and can steadily monitor the temp – important, since even a few degrees make a difference in the final texture of the caramels.