Prepare the bread: Spread butter on one side of each bread slice.
Layer the cheese: Place two slices, buttered side down, on a surface. Sprinkle ¼ cup mozzarella and ⅛ cup cheddar on each.
Add toppings: Arrange 8 pepperoni slices per sandwich. Spoon 1-2 tbsp marinara sauce over pepperoni, spreading evenly. Sprinkle Italian seasoning and garlic powder, if using.
Top with cheese: Add another ¼ cup mozzarella and ⅛ cup cheddar over the sauce. Place remaining bread slices, buttered side up, on top.
Cook: Heat a skillet over medium heat. Place sandwiches in the skillet and cook for 3-4 minutes per side, pressing gently, until golden and melty.
Serve: Slice diagonally and serve with extra marinara for dipping.
Air Fryer Option: Brush bread with olive oil instead of butter. Preheat air fryer to 360°F (180°C). Cook for 6-8 minutes, flipping halfway, until golden.