Place chocolate in a medium bowl. In a small saucepan over medium, bring cream just to a boil, then pour over chocolate and let stand 1 minute. Stir until smooth and shiny, then stir in butter. Let stand 10 minutes before using. If not using immediately, press plastic wrap directly on surface of ganache and refrigerate up to 2 days, or seal ganache (with plastic wrap over surface) with a tight fitting lid and freeze up to 1 month.