Help! I made a ganache last night to use under fondant. I used 14 oz Hershey's chocolate to 7 oz heavy whipping cream. I let it set out overnight and in the morning it's way too runny to ice a cake. Where did I go wrong? 

AWas your chocolate made up of 70% or more coco. This mades your ganache sets better. I think you used the right proportion but check your chocolate.

Oh dear, it's only 11%. No wonder it didn't work. Is there a way to thicken it?

No.  I had the same problem.  When it wouldn't set up I remelted it then put it back in the fridge and the next morning it was still semi liquid.  I know what I did wrong though.  Needed more chocolate.  I'm working on it.

A

Buy better quality chocolate, melt it and add it in. :)

When making ganache, always use very good quality dark eating chocolate of the European type (if you are in the USA)  the higher the Cocoa % the better the result.

What brand is the best? I only see Ghirardelli and Lindt at the store. I tried Lindt but it's way too bitter.

Perhaps try a 50-55% Cocao Dark Chocolate like Lindt ( I don't know which one you found too bitter but Lindt make up to 90% versions )  Does Ghiradelli make one like that?

You can also use Milk Chocolate, or part milk part dark but may need to use more of it.

I use Ghirardelli 60% because that's all I can get and it worked fine.  I have seen recipes using candy melts, you use less in the ratio otherwise the ganache will get rock hard when settting up and I don't think anybody wants the ganache too hard.

I got 10 Lindt 70% cacao bars. it cost me $25. Here's hoping this one works!

I measured it all by weight in ounces.

This has happened to me too ;-( thanks for the advice about 70% coco though ;-)

I use Colette Peter's recipe....12 oz semi-sweet (I use Ghirardelli) and 4 oz bittersweet, 1 1/2 cups of cream, 1 1/2 tsp vanilla.  Let sit overnight.  Works perfect for me every time.  Works as a filling and icing.  If too firm for spreading...just heat slightly.

*Peters'