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 10 more minutes, or until cheese is melted and pepperoni is a little crispy.