Welcome to the Cake Decorators Q&A

Hi,
I made Paul’s dark chocolate ganache for the first time yesterday. It was beautifully smooth when I put it in the fridge. I took it out today and since my house is pretty chilly I needed to microwave it a far bit to get it back to the right consistency even though it had been out of the fridge for a long while! I did it in short bursts as suggested but even after 30-40 seconds I had lots of tiny lumps. This I assumed was un-melted chocolate, so I again popped it in the microwave for just a few seconds at a time. I now have lump free ganache, but it is too runny to use. Today I was using it as a practice, ready for a wedding cake I am making at the end of the week, so all is not lost, but I am now not sure what to do with it. Would you normally just leave it to set a little outside of the fridge or would you recommend putting it back into the fridge to firm up?
So my questions really are two fold:
1) Why were there tiny lumps initially? Had I not melted it properly before I put it in the fridge? Was it still warm when I put it in (I don’t think so but maybe)
2) Did I simply over heat the ganache removing the lumps or did I possibly do something wrong with the recipe to affect the consistency?
3) Now the ganache is too runny what do I do? Wait for it to firm up or get more out of the fridge and put this back in to firm up properly?
Any suggests would be much appreciated.
Thanks