Place the chocolate and butter in a medium bowl. Heat the cream in a small saucepan over medium heat until it is very hot and just begins to steam. Pour the hot cream over the chocolate and butter and stir until the mixture is smooth. Add the vanilla and corn syrup; stir until the ganache is smooth and glossy. The ganache will be too soft to frost the cake immediately; it must sit at room temperature about 2 hours to firm up. The ganache can be made the night before the cake is assembled and left, covered, at room temperature. (Do not refrigerate it, or it will become too hard to handle.)