there is little to no diastatic power to a highly kilned malt like a highly roasted chocolate (most people would assume none). this means that you cant actually mash it with out combining it with base malt.
You can however, steep chocolate malt to get color and some flavor with out any appreciable extraction. But even this should be kept below 170-175 to avoid extraction of tannins and proteins from the malt, which will contribute astringency and cloudiness respectively to the final beer.
So in a nut shell you can't "mash" chocolate malt by its self, however there are other reasons not to steep it at temps over 170.
Hope this helps,
Chickens
