1. Preheat oven to 400 F.Toss sweet potato cubes with oil, chile powder salt & pepper, and spread in one layer on a baking sheet.
2. Bake for 20 minutes, or until golden brown.
3. Remove sweet potatoes from the oven and set aside.In a small food processor, blend together the yogurt, avocado, garlic, lime juice. Blend until smooth. Taste and adjust seasonings.Assemble tacos with roasted sweet potatoes, diced avocado, black beans, scallions and cheese.
4. Serve with avocado yogurt sauce.