Prepare the butter and salt in a small bowl so you're ready when you need to add it.
In a small saucepan, heat the heavy whipping cream and vanilla over medium heat until it just begins to simmer. Reduce the heat to low to keep it warm while you prepare the rest.
Heat the sugar and water in a separate small saucepan over medium heat, stirring occasionally with a wooden spoon until the sugar dissolves and it starts to boil, about 4 minutes. (I recommend between a 1.5-quart and 2.5-quart saucepan; any larger and you’ll have a hard time using the candy thermometer as it won’t full submerge in the caramel).
Stop stirring and clip a candy thermometer to the edge of the pot. Let the sugar boil without stirring at all, until it reaches 305°F, about 3-5 minutes. While it boils, grab a clean wooden spoon or rinse and dry the one you were stirring with before. (If you don't have a candy thermometer, for caramel sauce, it's okay - just boil until the sugar begins to turn amber, making sure to keep a close eye on it so the sugar doesn't burn. As soon as it turns amber, it will quickly darken, so immediately add the butter to cool it down).
Add the butter and fleur de sel, and slowly stir in the warm whipping cream. Be careful, as it will react and sizzle quite a bit when you start adding the cream. The temperature will also drop when you add the cream.
When the sizzling calms down, return to medium low heat and cook, stirring constantly, for 3 minutes.
Immediately transfer to a heat-proof bowl. If the caramel solidifies at all while you wait to serve, don't worry, you can just pop it in the microwave for 15-20 seconds and it'll soften up again.
Store any leftovers in an air-tight container in the fridge for about a month.
Notes
This recipe makes more than you'll need, but you won't be able to completely submerge the thermometer if you reduce the recipe, so keep any leftovers in a air-tight container in the fridge to serve over ice cream or these apple pie puffs. Just remove from the fridge and microwave for 15-20 seconds to soften up before drizzling.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.