Sue's Produce on 18th street has an amazing variety of potato called Korean sweet potatoes. They are purple outside, bright yellow inside, and are incredibly sweet and creamy. I think they taste like roasted chestnuts. They are only a dollar a pound. They don't always have a label on the box, but if you walk straight into the store all the way back you would hit your left thigh on them.
Let me know how you like them and if anyone tries making any recipes with them. Thank you.