I tried making ganache for the first time yesterday to fill a cake. I used Elisa Strauss' recipe (16 oz. chocolate and 5 1/2 oz. cream) which said it could be used to fill a cake. I used Lindt chocolate and cream from a local dairy, so they were both good quality ingredients. The ganache melted perfectly and thickened after sitting at room temperature for a couple hours. Then I put it in a plastic container to use later that night (the recipe said it could be stored at room temperature for 2 days).

When I went to fill my cake, I discovered that it had hardened into an unspreadable mass. It wasn't as hard as a chocolate bar, but I definitely couldn't spread it on my cake or pipe it with my pastry bag. I kept water as far away from it as possible, so I don't think it seized.

I melted it again over a double boiler and let it cool. Since it was 8 p.m., and I needed to get decorating, I set the bowl on an ice pack and stirred to help it cool faster. I finally got it a nice, thick consistency and then all of a sudden it hardened into the unspreadable mass again

What did I do wrong?! Thanks in advance for your help!

Ganache will harden and in fact is supposed to, I believe. I always just microwave mine in short intervals at 50% power until I get it the consistency I'm looking for. The chocolate to cream ratio will also affect the consistency. I have one recipe that calls for 8 oz. semi-sweet chocolate and only 2 TBSP. cream. You have to spread this quickly and then have a sharp knife to cut the cake but it gives a nice, unexpected "crunch" of goodness. Don't despair; just keep experimenting.

The ratio you are using is wrong for spreadable ganache. You need 2 (chocolate) : 1 (cream) for dark chocolate and 3:1 for milk and white chocolate. So you should have used 16oz chocolate : 8 oz cream. Just remelt what you have, add the extra cream, mix it in and let it cool and it will be fine.