Make the icing while the cake is baking because it needs to thicken for 30 minutes before use. (If it is too runny, it will pour off the sides and look messy.) Put the chocolate and butter into a heatproof bowl. Place the cream and corn syrup in a saucepan and stir over medium heat until they come just to a boil. Pour immediately into the bowl, stirring until the chocolate and butter have completely melted, and the mixture is thick and glossy. Set aside.