In a large bowl, combine chocolates, butter, and salt.

In a medium saucepan, heat cream over medium-low heat until it just begins to steam. (Do not boil.) Pour warm cream over chocolate mixture, and let stand for 3 to 5 minutes. Starting at center of bowl, slowly stir in small circles with a rubber spatula until well combined. (This ensures you are not incorporating too much air into the ganache.) Let stand at room temperature until thick and spreadable, 1½ to 2 hours.