Preheat your oven to 400°F (200°C). Toss the cubed sweet potatoes with 1 tablespoon of olive oil, cumin, chili powder, salt, and pepper. Spread them in a single layer on a baking sheet.
Roast the sweet potatoes for about 20-25 minutes, tossing halfway through, until they are tender and caramelized around the edges, filling your kitchen with fragrant spice aromas.
While the sweet potatoes roast, heat 1 tablespoon of olive oil in a skillet over medium heat. Add the rinsed black beans and cook for 5 minutes, stirring occasionally, until heated through and slightly softened.
Once the sweet potatoes are done, remove from oven and let them cool slightly. Then, gently mash some of the sweet potatoes with a fork to create tender chunks and caramelized bits.
Warm the tortillas in a dry skillet over low heat until soft and pliable, about 30 seconds per side, or wrap in foil and warm in the oven.
To assemble, pile some mashed sweet potatoes on each tortilla, followed by the black beans. Top with chopped cilantro if using, and squeeze lime juice over for added brightness.
Fold or roll the tortillas into tacos, then serve immediately, garnished with extra lime and cilantro if desired.