Separated, grainy ganache (Dark and Stormies from Grewelings's book)
I don't know the details of this particular recipe, so you'll have to excuse me if this comes across as a bit of a shot in the dark, but here are a few things that could have gone wrong:


Grainy chocolate is usually a sign of seizing.  White chocolate still contains cocoa butter and can still seize.  Therefore it's important not to let any liquid touch the melting chocolate and to not let the temperature get too high.
130-140° F is definitely way too high.  White chocolate will normally burn or seize at temperatures higher than around 110° F.  You mentioned that the second time you used the right temperature, but it's worth pointing out anyway: Be very careful with the temperature, don't use direct heat preferably, use a double boiler or a stainless steel bowl placed over a steaming pot, and stir frequently to keep the temperature even.
Don't dump hot cream into the chocolate.  It's strange that almost every recipe tells you to do this; water causes melted chocolate to seize, period.  The only way to avoid this is to use a very large amount of liquid for a very small amount of chocolate, so what you have to do is go the other way; incorporate the chocolate into the cream, a small amount at a time.  This is especially important with tempered chocolate because you've essentially raised the melting point!
Also be careful not to let any water get into the chocolate as it's melting; use dry utensils and make sure you don't have any steam condensing over top (use a large bowl over a small pot if you don't have a double boiler).
Finally, as Joe commented, make sure you're using the right kind of white chocolate.  I've never seen baker's white chocolate, so when I need white chocolate for melting I generally use the white chocolate chips.  If yours didn't burn at temperatures as high as 130° F then you might have been using coating chocolate instead.


Follow all those precautions and you should end up with a very smooth mixture.  I've done this for ganaches and even foams and it's never a problem if you're careful about both the temperature and moisture.