Cut the bread lengthwise and widthwise without cutting through the bottom crust. This can be a little tricky going the second way but the bread is very forgiving.
Place bread on a foil-lined baking sheet and insert cheese slices and pepperoni between cuts. Combine butter, garlic powder, and Italian seasonings. Drizzle or brush over bread with a pastry brush. Sprinkle bread with Parmesan cheese.
Wrap bread completely in foil and bake at 350 degrees for 15 minutes. Unwrap the bread and bake 5-10 more minutes, or until cheese is melted and pepperoni is a little crispy.