Preheat oven to 400°. Preheat grill on high.
Open buns, spread the cut side of each half with butter. Sprinkle evenly with 2 tablespoons parmesan cheese and garlic powder. Toast in the oven for 5 minutes or until golden brown. Remove from oven and set aside.
Cut four 1/4-inch slices from the mozzarella ball. Break the remaining cheese into smaller pieces. Reserve.
In a large bowl, combine ground beef, oregano, remaining 1/4 cup parmesan cheese, 1/2 cup pepperoni, salt, pepper and 1/4 cup of marinara sauce. Mix until well combined; divide into 8 portions.
Form into patties, top 4 with a slice of mozzarella cheese. Cover with remaining patties and pinch edges together until all seams are closed. Shape into a uniform round patty.
Grill patties for 7 minutes on each side. Remove from grill and place on a baking sheet. Finish cooking in the oven for 10-15 minutes or until internal temperature reaches at least 130°. Top each patty with remaining marinara sauce, mozzarella cheese and pepperoni. Return to oven and bake until cheese is melted.
Remove from oven, serve on toasted buns.
Along with the cheese and pepperoni, try topping the burgers with more great pizza toppings such as bell peppers, mushrooms or onions.