Have you tried boniatos yet? They are commonly called Cuban sweet potatoes, or sometimes white sweet potatoes. I've tried them a couple of different ways now, and I've got a few tips for you. I was told to roast them in their jackets just like you would a regular sweet potato, and that worked quite well, except that they seemed to take just a bit longer to fully soften.
And then I decided to experiment. Although I generally recommend experimenting, this one did not turn out very well. I peeled them first, intending to boil and mash them. The boiling and mashing went ok, but the peeling was very very disconcerting: boniatos oxidize RAPIDLY, leaving the exterior flesh grayish. Not at all appealing, no pun intended. I don't think it affected the taste, but they were not pretty!