Preheat oven to 350°. In a medium bowl, combine ground beef, garlic and parsley and mix with spatula until just combined. Using your hands, form 4 equally-sized patties; season both sides with salt and pepper.
Heat oil in a large skillet over medium-high heat. Add burgers and cook, 4 to 5 minutes for medium. Flip patties, then immediately pour 2 cups marinara around them in pan. Reduce heat to medium-low and simmer for another minute, then top each with cheese and pepperoni slices. Cover pan and cook until cheese is melty and burger is cooked to your liking, about 3 minutes more.
Meanwhile, make garlic buns. Brush melted butter on inside of each half of the rolls and sprinkle with garlic powder, Italian seasoning, and Parmesan. Bake until golden, about 10 minutes.
Spread remaining 1/2 cup marinara on bottom buns followed by cooked burgers and the top bun halves.