You can use any kind of sweet potato you like (or even mix in some taro) as long as they're not the orange-flesh ones often referred to as "garnet yams" or "jewel yams" because they are too wet. If you can get the really dark purple sweet potatoes, they make the most beautiful purple doughnuts! Make sure you eat this while they're fresh as they don't keep well. The dough can be made in advance and kept in the fridge for a few days, so you can fry up only as much as you can eat in 1 sitting.