Preheat the oven to 425 degrees. Toss the prepared sweet potatoes with olive oil, cayenne pepper and salt. Arrange in a single layer on a large baking sheet and bake for 30 to 40 minutes, tossing halfway.
Prepare the black beans:
Warm the olive oil in a large saucepan over medium heat. Add the onions and a sprinkle of salt. Cook, stirring occasionally, until the onions have softened (about 5 minutes). Add the cumin and chili powder and cook for about 30 seconds while stirring. Pour in the beans and 1/3 cup water. Stir, cover and reduce heat to a simmer. Cook for 5 minutes, then remove the lid and use the back of a fork or potato masher to mash up at least half of the beans. Remove from heat, stir in salt and pepper, and cover until you're ready to serve.
Assemble the tacos:
Spread black beans down the middle of the tortilla, top with sweet potatoes and guacamole. Garnish with cheese and fresh cilantro.