Spread butter on one side of each slice of bread.
On the unbuttered side, spread pizza sauce evenly.
Sprinkle shredded mozzarella cheese and Parmesan cheese.
Add pepperoni or other toppings.
Close the sandwich with another slice of bread, buttered side out.
Heat a skillet over medium heat and grill each sandwich until golden brown and the cheese is melted, about 3-4 minutes per side.