How to Make It
In a medium saucepan, combine both of the sugars with the syrup and salt. Cook over moderate heat, stirring occasionally, until the mixture is molten and beginning to bubble at the edges, about 6 minutes. Simmer, stirring, for 1 minute more, then stir in the butter. Attach a candy thermometer to the side of the pan and cook the sauce until it reaches 240°, about 2 minutes.
Carefully stir in the cream until incorporated and bring to a rolling boil. Remove from the heat and stir in the vanilla, lemon juice and 1 1/2 tablespoons of water.
Let the sauce cool completely, then transfer to a container and refrigerate. Rewarm in a microwave before serving, or serve at room temperature.