In a small saucepan or in the microwave oven, bring the cream to a boil. Pour over the chocolate. Let stand for 1 minute without stirring.
With a whisk, stir to emulsify until smooth. Add the butter and stir until it has completely melted. Refrigerate for about 1 hour or until the ganache is spreadable but not too thick. If necessary, melt for a few seconds in the microwave.