12 tablespoons unsalted butter, at room temperature and cut into pieces.
1 cup heavy cream, at room temperature.
1 tablespoon fleur de sel (or another sea salt)
Mix the brown sugar, half-and-half, butter and salt in a saucepan over medium-low heat. Cook while whisking gently for 5 to 7 minutes, until it gets thicker. Add the vanilla and cook another minute to thicken further. Turn off the heat, cool slightly and pour the sauce into a jar.