In a medium saucepan, combine the cream and corn syrup. Place over medium heat and bring to a simmer. Remove from the heat and add the chocolate and butter. Shake the pan and allow to stand for 3 to 4 minutes, then whisk smooth and whisk in the liqueur, if using.
Scrape the frosting from the pan to a bowl to cool and thicken. To use immediately, stir it over a