Spread butter on one side of each bread slice.
Place one slice of mozzarella on the unbuttered side of two bread slices.
Add pepperoni and a spoonful of marinara sauce.
Sprinkle with Italian seasoning, then top with another mozzarella slice.
Cover with the remaining bread slices, buttered side up.
Heat a pan over medium heat and grill the sandwiches until golden brown on both sides, about 3-4 minutes per side.
Serve hot with extra marinara sauce for dipping.