If this doesn’t set chocolate lovers’ teeth to aching, we don’t know what will. Combine the chocolate, cream, corn syrup, and sugar in a small heavy saucepan. Over low flame, heat, stirring, until the chocolate is melted and the sugar is dissolved. Raise heat to medium and, stirring only enough to prevent sticking, cook until the mixture reaches 224 degrees F to 226 degrees F on a candy thermometer or forms a soft ball when dropped into water. Then, using a small wire whisk, beat the hot mixture into the egg. Cool to room temperature and stir in the vanilla. Makes ample for a double-layer 9-inch cake.