Melt butter in a large skillet over medium heat. On one half of a tortilla layer 1/4 cup cheese, then half the spinach and black beans, plus a sprinkle of salt. Spread another 1/4 cup cheese over that and fold the tortilla over. Carefully transfer to waiting skillet and cook until golden brown, about 3-4 minutes. Flip and cook another 2-3 minutes until that side is golden brown as well. Repeat with the remaining ingredients, then top with avocado and sour cream and eat immediately.