Heat over medium-high heat, whisking it regularly. The sugar will form clumps and puddles, but just keep whisking it until all of it has melted.
Once all of the sugar has melted, stop whisking it.
Keep cooking the sugar until it reaches a deep golden amber color. Watch the sugar closely as it could easily overcook.
Once the caramel is deep golden amber, add the butter, and whisk until it is completely incorporated. The caramel will bubble up, so be careful.
3/4 cup salted butter
Remove the pan from heat.
Slowly stream the cream into the caramel, whisking to combine. The caramel will bubble up again, so be careful.
1 cup heavy cream
Once the cream has been incorporated, whisk in the vanilla and fleur de sel.
1 tablespoon vanilla extract, 1 teaspoon fleur de sel
Let the caramel sit for 10-15 minutes.
Pour into a glass jar, and let cool to room temperature (uncovered).
The caramel can be refrigerated for up to 4 weeks.
Warm before using.
Video
Notes
Salted butter: I've tried using unsalted butter and salted butter in this recipe. I prefer the flavor with salted butter because it's more balanced and less sweet.
Fleur de sel: Fleur de sel is a finishing salt that's used to "finish" dishes. It has larger grains and is less "salty" than table salt. The best substitute will be a good-quality large-grain sea salt flake. Maldon sea salt in one substitute. If you can't find fleur de sel or Maldon sea salt, kosher salt would be the best choice.