Cut bread loaf lengthwise and widthwise (without cutting all the way through the bottom of the loaf), creating approximately 1-inch wide sections of bread.
Place bread on a foil-lined baking sheet, stuffing the shredded mozzarella and pepperoni between the bread slices.
In a small bowl, combine melted butter, garlic powder, and pizza seasoning. Using a pastry brush, apply a light coat of the mixture all over the bread (save for about 1/4 of the butter mixture).
Lightly spray foil with non-stick spray and wrap bread completely in foil. Bake at 350 degrees for 15 minutes.
Remove from oven and unwrap the bread. Add the remaining butter and sprinkle all over with Parmesan cheese.
Return to oven (bread unwrapped) and bake 10 more minutes (until cheese is melted and bread starts to look golden).