Preheat your oven to 400°F. Pierce the sweet potatoes several times with a fork, place them on a baking sheet, and roast for 45-50 minutes until very tender when pierced with a knife.
Let the sweet potatoes cool until safe to handle, then scoop the flesh into a medium bowl, discarding the skins. Mash the flesh with a fork until smooth.
Stir the olive oil, chili powder, cumin, smoked paprika, garlic powder, onion powder, salt, and black pepper into the mashed sweet potato until the spices are fully incorporated.
Lay the tortillas on a clean work surface. Spread an even layer of the spiced sweet potato mixture over one half of each tortilla, leaving a 1-inch border. Sprinkle the Monterey Jack and cotija cheeses evenly over the sweet potato.
Fold the bare half of each tortilla over the filling to create a half-moon shape, pressing down gently.
Melt 1/2 tablespoon of butter in a large skillet or griddle over medium heat. Cook one quesadilla for 2-3 minutes per side, until the tortilla is golden brown and crisp and the cheese is melted. Repeat with remaining butter and quesadillas.
Transfer the cooked quesadillas to a cutting board, let them rest for 1-2 minutes, then slice each into three wedges. Serve immediately with sour cream and a garnish of fresh cilantro.
