Place rack in the upper-middle position in the oven. Preheat the oven to 450°F. Line a rimmed baking sheet with foil.
Cut the loaf of bread in half crosswise, and then cut each piece in half horizontally to make 4 equal pieces.
Place the bread cut-side down on the prepared baking sheet. Brush the crust with the olive oil.
In a small bowl, whisk together melted butter, garlic powder and salt. Flip the bread cut-side up and brush the cut side with the melted butter mixture.
Bake the bread (cut-side up) until it’s browned on the edges, about 5 minutes.
Spread sauce evenly on each piece of toasted bread. Top with mozzarella, then Parmesan, and finally the pepperoni or other optional toppings (in that order). Return to the oven and bake for about 10-15 minutes, or until the cheese is melted and starting to brown.
Let the pizza rest for about 5 minutes before serving.