Preheat the oven to 400°F. Toss the sweet potato cubes with 1 tablespoon olive oil, cumin, chili powder, and salt on a baking sheet. Spread them in a single layer and roast for 20 minutes until tender and lightly browned.
While the sweet potatoes roast, heat the remaining 1 tablespoon olive oil in a skillet over medium heat. Add the black beans and cook for 5 minutes, stirring occasionally, until warmed through. Season with a pinch of salt if needed.
Warm the corn tortillas in a dry skillet or microwave for 20 seconds until pliable.
Fill each tortilla with a scoop of roasted sweet potatoes and black beans. Place the sour cream in a small zip-top bag, snip off one corner, and pipe concentric circles on top of each taco. Drag a toothpick from the center outward to create the spiderweb pattern.