In a saucepan, bring the cream and corn syrup to a boil. Pour over the chocolate and let melt for 2 minutes without stirring. With a spatula, stir the ganache until smooth. Add the butter and stir until melted and smooth.
Let cool for 2 hours at room temperature or until the ganache is spreadable. Serve or cover with plastic wrap.
The ganache will keep for 3 to 4 days at room temperature or in a refrigerator. Let come to room temperature and mix thoroughly before serving.
Depending on the percentage of cocoa mass in the chocolate, the ganache will be more or less firm.