Preheat the oven to 350° F(175°C). In a bowl, mix together the melted butter, parsley, garlic, Parmesan salt, pepper, and oregano.
Make 2 cuts along the top of the buns. Stuff 2 pieces of pepperoni in each slit, along with a pinch of Mozzarella. Place the buns on a baking sheet lined with parchment paper.
Brush the butter mixture over the tops of the rolls.
Bake for 10 minutes. Brush on more butter and serve hot with marinara sauce on the side for dipping.
Notes
When slicing the rolls, cut only halfway through to keep them intact, as they can break easily. If you find them difficult to work with, try warming them slightly to make them more pliable.