Preheat your oven to 400°F (200°C). Pierce the sweet potato several times with a fork, place it on a baking sheet, and roast for 45-60 minutes, or until it is very tender and easily pierced with a knife.
Let the sweet potato cool until it is safe to handle, then slice it open and scoop the flesh into a medium bowl. Mash the flesh with a fork until it is mostly smooth.
To the mashed sweet potato, add the olive oil, chili powder, cumin, smoked paprika, and salt. Stir until the spices are fully incorporated.
Lay the tortillas on a clean work surface. Spread about 1/2 cup of the sweet potato mixture evenly over one half of each tortilla, leaving a 1-inch border around the edge. Top the sweet potato layer with 1/4 of the black beans and 1/4 cup of the shredded cheese.
Fold the other half of each tortilla over the filling. Lightly spread the softened butter over the top outer surface of each quesadilla.
Heat a large skillet or griddle over medium heat. Carefully place one or two quesadillas in the skillet, buttered-side down. Cook for 3-4 minutes, until the bottom is golden brown and crisp.
Carefully flip the quesadillas and cook for another 2-3 minutes on the second side, until golden brown and the cheese inside is fully melted.
Transfer the cooked quesadillas to a cutting board, let them rest for 1 minute, then slice each one into three wedges. Serve immediately with a dollop of sour cream and a sprinkle of fresh cilantro.
