This is a great salad to make with leftover roasted sweet potatoes but you can also roast them just to make the salad. The trick to succeeding with crispy kale is to make sure it is completely dry before you put it in the oven. If you are using bunched kale I recommend that you stem and wash it, spin it twice in a salad spinner, then set the leaves in single layers on a few layers of paper towels and roll them up. You can then refrigerate for up to a day or two. Once the salad is assembled, the portion of kale that you toss with the sweet potatoes will soften, and the kale that surrounds the sweet potatoes will remain crispy.