Why does my ganache keep breaking?
I found an answer in McGee's 'On Food and Cooking: The Science and Lore of the Kitchen'. 

The basic ganache is 1:1 chocolate:cream (by weight). With lots of chocolate the emulsion can come apart.

In 'Keys to good cooking' McGee describes how to restore a failed ganache. You put it over a double boiler and when it reaches 33ºC y stir it vigorously. If that fails, start with a smaller batch, just like you'd recover mayonnaisse.

McGee recommends letting it sit on the counter to cool slowly overnight.

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