Preheat oven to 400° F and line a large baking sheet with parchment paper.

Place the sweet potatoes on the baking sheet. Drizzle with olive oil and toss with the chili powder and pinches of salt and pepper. Spread in a single layer on the baking sheet. Roast for 20 minutes, or until puffed, fragrant, and golden brown.

Make the avocado yogurt sauce: In a small food processor, combine the yogurt, avocado, garlic, lime juice, and a few generous pinches of salt and pepper. Pulse until smooth. Taste and adjust seasonings. Chill until ready to use.

Assemble the tacos in the tortillas with a scoop of the sauce, the roasted sweet potatoes, black beans, and desired toppings. Season to taste and serve with lime wedges for squeezing.