1 cup packed brown sugar

½ cup heavy cream

¼ cup unsalted butter

1 tablespoon vanilla

Pinch salt

Mix the brown sugar, cream, butter and salt in a saucepan over medium-low heat.  Cook while whisking gently for 5 to 7 minutes, until mixture thickens.  Add the vanilla and cook for another minute, or until the mixture thickens further.  Turn off the heat and cool.