Place the chocolate in the top of a double boiler, over simmering water. (Or a bowl that will fit over a saucepan without the bottom touching the simmering water underneath)  Melt slowly, stirring occasionally.  Once the chocolate has melted place the top of the double boiler directly on the burner, over low heat, and whisk in the sugar, butter, salt and single cream.  Cook and whisk to combine and melt the butter.  Increase the heat to medium and cook for a further 4 minutes or so (without allowing the mixture to boil) until the sauce thickens.  Remove from the heat and whisk in the vanilla.  Serve warm or at room temperature.