Preheat oven to 350°F (175°C). Spray a baking sheet with cooking spray. Clean mushrooms with a damp paper towel. Carefully break off stems. Chop stems extremely fine, discarding tough end of stems.
When garlic and mushroom mixture is no longer hot, stir in cream cheese, Parmesan cheese, black pepper, onion powder and cayenne pepper. Mixture should be very thick. Using a little spoon, fill each mushroom cap with a generous amount of stuffing. Arrange the mushroom caps on prepared cookie sheet.
Bake for 20 minutes in the preheated oven, or until the mushrooms are piping hot and liquid starts to form under caps.