Laurie - check out Dagoba, it's an organic chocolate company that makes a bar called the Eclipse. 87% and 170 cal/oz. It's good, and you're right - darker the better in terms of heat resistance.
The Eclipse, for you dark choco connoisseurs out there, is a reeeally good bar. There are some that are as dark or darker, but this is about as dark as you can go without getting overly bitter. The Eclipse is a lot smoother than many darks in the 70s.
The downside is that it's about a buck and a half an ounce, and my backcountry diet "requires" about 6 oz/day of chocolate...
They also make the 100% Prima Materia but it's...an acquired taste. I do like it better now than I did six months ago.