Carefully cut off stems of mushrooms. Use a spoon to help remove most of stem area so you have a large area of the cap to stuff. Place caps onto a large greased baking sheet.
Discard any tough ends of stems and finely chop remaining parts.
In a large skillet, heat 1 Tablespoon of the butter over medium heat. Add chopped mushroom stems to skillet. Cook stems until tender and any moisture from the mushrooms and butter has disappeared. Set aside to cool.
When mushroom stems have cooled, stir in cream cheese, mozzarella cheese, 1 Tablespoon Panko breadcrumbs, 1 Tablespoon Parmesan cheese, garlic powder, black pepper, onion powder, and ground cayenne pepper.
Using a small spoon, generously fill each mushroom cap with cheese mixture.
For Topping: In a small microwave-safe bowl, melt the remaining 1 tablespoon butter. Then stir in the remaining 1 1/2 tablespoons of Panko breadcrumbs, 1/2 tablespoon Parmesan, and 1 teaspoon parsley. Evenly distribute mixture over the tops of the stuffed mushrooms.
Bake at 350 degrees F for 20-25 minutes, until cheese has melted and topping starting to brown.