Also known as Japanese sweet potatoes, these tubers have deep magenta skins and a white flesh that turns champagne gold when cooked. They're simultaneously drier and creamier than orange-fleshed sweet potatoes, which means that when you roast them at high heat (cubes, wedges, circles, or halves, simply coated with olive oil and salt and cooked between 425 and 450°), their edges get unbelievably crispy and burnished, belying the pudding-like centers, which taste chestnutty rich and sweet. Make into oven fries; steam; or bake and fill for an almost cheesecake like dish.