Cut sweet potato into cubes or thick slices.

In a skillet add 2 tablespoon olive oil and add sweet potato, garlic powder, salt and barbecue spice and cook for about 8-10 minutes or until sweet potatoes are soft and cooked well.

In another skillet, add rest of olive oil and onion, garlic, cook over low heat until the onion is translucent, about 8minutes.

To this add cooked black beans, corn, cumin, chili/cayenne pepper, and salt to taste. Cook over medium-low heat until heated through, 5 to 7 minutes. Remove from the heat.

Warm the tortillas one at a time for about 20 seconds on each side in a dry skillet set over medium heat. Or, if you have a gas stove; place a tortilla straight over the flame for a few seconds on each side. As you heat the tortillas, stack them on the damp towel and cover the tops of them with the towel to retain moisture.

Place the avocado in a small bowl and use a fork to gently mash it.

To form the taco, spread some avocado on half of each tortilla. Spoon some beans and sweet potato on top, and then add the tomato and cilantro. Drizzle with some lime juice and sour cream. Fold each tortilla in half. Serve at once.