Honestly, I don't really know what a yam is. I've read (thank you, Wikipedia) that the sweet potato is not even related to the yam, even though Americans tend to use the names interchangably. I've never even seen a yam in the grocery store. When a recipe calls for yams, I use sweet potatoes — like in this dish, Farmhouse Chedda and Yam Soup from The Uncheese Cookbook:
I'm not sure how it would have tasted with actual yams, but it was delectable made with sweet potatoes. Remember that Campbell's condensed cheese soup in a can? I used to be obsessed with that stuff when I was a kid. This is kind of like that, only with far more complex flavors and none of that funky processed canned soup crap.
It's made with yams (or sweet potatoes), nooch, carrots, onions, and green pepper. But everything is pureeed, so the veggie flavors meld together and get all creamy. This was perfect for a cold winter night, served alongside crusty whole wheat garlic bread and a salad with vegan ranch and faux bacon bits.