Step 1 Place the cream, sugar and corn syrup in a heavy bottomed, non-reactive saucepan and slowly bring to a boil. Remove from the heat and whisk in the chopped chocolate until it is completely melted and smooth. Whisk in the butter, one piece at a time, and pour into a bowl to cool completely. (At this point, the ganache may be kept covered and refrigerated 3 days.)
Step 2 If refrigerated, allow the icing to return to room temperature and stir until smooth before icing the cake.