Preheat oven to 400ºF.
Toss sweet potatoes in olive oil, chili powder, salt and pepper.
Spread into a single layer onto a baking sheet.
Roast in the oven for 10-15 minutes until golden, and set aside to cool slightly.
Add roasted sweet potatoes to a medium size bowl.
Mix in beans, scallions, avocado, and jalapeño. Season with salt and pepper.
Assemble the tacos and top with cotija cheese and avocado sauce.