Part of the reason that chocolate is so irresistible is the way that it is made up of a variety of fats, both from the cocoa bean and in many cases milk or even vegetable. These fats happen to melt at just below body temperature so the chocolate melts in your mouth. This is why chocolate is so obviously a bad material for making teapots, hence the phrase "as useless as a chocolate teapot"!
However, James called in to the show to ask how thick the walls of a chocolate teapot would need to be to