½ cup unsalted butter, at room temperature
1 cup dark brown sugar
1 cup heavy cream
1 teaspoon fleur de sel (or other flaky sea salt)
1 tablespoon vanilla extract
1. Melt the butter in a medium-sized saucepan over medium heat. Add the sugar, cream and salt, and whisk until well blended. Bring to a boil then reduce the heat slightly and gently boil for 5 minutes.
2. Remove from the heat and whisk in the vanilla extract. Serve warm or at room temperature (the sauce will thicken as it cools). Store in an airtight container in the refrigerator and re-warm before using.