Gather all of the ingredients for this recipe and have them measured out. It moves fast and has the tendency to burn.

In a heavy-bottom saucepan, melt the butter over medium heat.

Just before it is fully melted and starts to simmer, add the dark brown sugar and stir with a wooden spoon until it forms a paste. Stir in the salt.

Allow the mixture to bubble without stirring. It will start to look more smooth and creating larger bubbles, approximately 3-5 minutes.

Reduce the heat to low and pour the cream into the sugar mixture while whisking. It might spit and sizzle and the sugar might clump, this is all normal.

Simmer until it is a thin syrup, about 5-8 minutes, whisking every 2 minutes. If using a candy thermometer, heat until it reaches 225°F, this is right before it reaches the "thread" stage of candy making.

Remove from the heat and whisk in the vanilla and an additional punch of fleur de sel, if using.

Use hot or wait until fully cooled before canning or storing in the refrigerator.

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