Heat a medium skillet over medium heat and fry the pepperoni until crisp. Transfer onto a paper towel to drain.
Brush each tortilla with a thin layer of pizza sauce. You want the layer so thin that if you turned it over, none would drip.
Sprinkle cheese on top of the sauce on the bottom tortilla. Top with pepperoni and other toppings, if desired. Sprinkle with another layer of cheese and place the other tortilla on top (sauce side in).
Preheat a medium cast-iron skillet over medium heat. Add a little butter if desired and melt.
Gently place quesadilla in the pan and cook for 3-5 minutes on each side, until cheese is melted and tortillas are crispy.
Slice into quarters and serve with a little bowl of warm pizza sauce for dipping.