Mix sugar and milk and slowly bring almost to a boil. In the meantime melt chocolate in the double broiler the old fashioned way, or microwave it at about half power level until the shape of the squares are modestly discernible. The use of full power and over-cooking can result in burnt chocolate.

Add the melted chocolate and coffee to the hot milk and sugar. Mix and cook till the temperature [using a candy thermometer] reads 218 to 220 degrees. Briefly stir in butter and let cool to room or slightly warm temperature and beat in vanilla.

This is not a last minute operation; the cooking time of the sauce seems interminable [40 to 50 minutes] and the cooling time is another half hour.