Preheat oven to 375 degrees F. Gently clean mushrooms by wiping them with damp paper towels. Remove stems from 12 mushrooms; set aside caps. Finely chop mushroom stems and remaining whole mushrooms.
In a medium skillet over medium heat, melt butter; saute chopped mushrooms 3 to 4 minutes, or until tender. Remove from heat and add bread crumbs, pepperoni, oregano, garlic powder, salt, and pizza sauce; mix well.
Using a teaspoon, stuff mushroom caps with stuffing mixture. Place on baking sheet.
Bake 15 minutes. Remove from oven and sprinkle each with mozzarella cheese. Return to oven 5 minutes, or until cheese is melted and mushrooms are heated through. Serve immediately.
