Combine brown sugar, 1/2 & 1/2 cream, butter & salt in a saucepan over medium-low heat. Cook while whisking gently for a couple of minutes, until it slightly thickens. Add the vanilla & cook another minute. You are not cooking the sauce very long so it remains quite pourable. Turn off heat, cool slightly. Pour sauce into a blender & blend on high until sauce is ‘creamy’. Refrigerate until cold.