Make sure to heat the cream for the ganache so that it's very hot without scalding it. You should be able to heat it for 1 minute without any problems but be sure to stir it well at that point and then test the temperature. You will likely need to heat it more but I would reduce the time to about 30 seconds and be sure to stir it well after each microwave session so that you don’t end up with areas that are scalded.