If it seems that
the temperature of the mixture is rising very slowly, cover the bowl with the lid of the pot, as it sits over the simmering water.
The ones I popped in the fridge overnight did not, so I am guessing that it is all about
the temperature of your mixture.
The temperature of the mixture should not exceed 113 °F / 43 °C, which is why the cream is added in several additions.
Make sure to use a candy thermometer when you make it, though, because if
the temperature of the mixture gets too high it will become crunchy.
Primary fixed points were
the temperatures of a mixture of common salt and ice and the temperature of the human body; with reference to these the freezing point of water was marked 32 degree and the boiling point of water was marked 212 degrees.
4 As the chocolate melts, allow
the temperature of the mixture to fall to the cooling temperature.