In a medium bowl, mix together the cream cheese, parmesan cheese, black pepper, onion powder and cayenne pepper. Once combined, fold in your mushroom stem and garlic mixture.
Place the mix into a plastic baggie and snip off one corner. Fill your mushroom caps generously with the cheese mixture.
Bake for 20 minutes or until the mushrooms are tender. When they are done, a small amount of liquid will start to form under the mushrooms.