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.

I don't think I've ever even had hot buttered rum, but I can take a stab at some of your questions:

• 1 part ice cream and 2-3 parts boiling water will still average out pretty hot. If the ice cream is at, say, -20 to -10C, that'll get you in the 60-70C range initially, which is I think fairly normal for hot drinks. And of course, if you keep it in the refrigerator instead, your drink will be hotter!

• I suspect mixing ice cream with butter will help it all mix together better in the end, by helping disperse the fat.

• Ground spices aren't going to need a ton of steeping. I imagine the boiling water would be enough, and if not, the pre-cooking could easily compensate. And since some recipes just have nutmeg sprinkled on top, it sounds like steeping isn't necessarily the goal. If you're really aiming for spiced hot buttered rum, you'll probably want more spices than those recipes, and perhaps to cook them briefly with the butter.

• If you intend to freeze the "batter", including ice cream will help keep it soft enough to scoop/slice. If it were just butter and sugar, it'd get quite hard in the freezer.

• Yes, the milk/cream content of the ice cream would make the drink a little creamier - but I don't know whether hot buttered rum should be a bit creamy. It sounds like it'd be fine either way - personal preference, perhaps?