http://www.imperial-library.info/content/troll-slaying just implys the fat is all around the body. Stomach, arms and legs mostly, I'd assume.
Let the flames die down and then examine the troll's corpse. If you're lucky, you'll find some fat deposits that will fetch a good price in an apothecary's shop. In fact, if you've got a knack for alchemy yourself, you can boil the fat down for use in all manner of potions and tonics