Mix tuna, black beans, and sour cream together in a large bowl. Stir corn, buffalo wing sauce, garlic salt, and cumin into tuna mixture.
Melt half the butter in a large skillet over medium heat; heat 1 tortilla in melted butter for 1 minute and flip. Spread half the tuna mixture over heated tortilla. Sprinkle with half the Mexican cheese; cover with another tortilla. Place a lid on the skillet and cook for 1 minute.
Flip the quesadilla, cover, and cook until cheese is melted, 1 to 2 minutes. Repeat with remaining butter, tortillas, and filling. Slice quesadilla into eighths and serve a half quesadilla to each guest.
