Uh oh. Sounds like one of two things happened: 1. Your chocolate might have seized while melting it. To prevent this, keep the top bowl of the double boiler away from the water. I also recommend that you use my hybrid approach to the double boiler and only place the bowl on top of the water when it is off the heating element. This will help to prevent overheating, which can cause the chocolate to seize. I also recommend that you add a bit of room temperature cream to the bowl with the chocolate, as this seems to help the melting process. 2. You might not have given your ganache ample time to cool prior to adding it to the whipped cream. If hot chocolate comes in contact with cold cream, it will cause the chocolate to seize. Be sure to allow your ganache to cool completely before proceeding with the recipe.